There are plenty of websites that will tell you to do a fresh installation of Windows - say - every few years, to clear out the garbage that accumulates over time.
However, I've always been under pressure from my family to keep all the old programs on their PCs, and often it's very hard to find all of the original downloads, serial numbers and source disks to re-install everything, so keeping the old installation is a good solution if you can make it work.
Today, with Windows 10 having painless upgrades from Windows 7 and 8, this isn't the problem that it once was. It was once much harder.
Over time, I collected some knowledge about how to do it. It's turned out to be not too hard after all. The result is that several of my family's computers have a history going back as much as 15 years (to Windows 95, which is as far back as you can easily get) - and all the software works.
The result is Grandfather's Axe - I presume you know the story? I've still got grandfather's axe, it's the best one I've ever had, it's only had four new heads and three new handles over the years. (If you don't get the joke - axes consist of only the head and the handle - so it's been completely replaced several times over.) What I've done with these computers is like that - new motherboards and new operating systems - but the programs are still there. Here's how it all works.
(1) Garbage - you can get rid of garbage without re-installing, CCleaner is pretty good at that and it's free. Running on-line malware and virus checkers helps too.
(2) Windows 95/98/2000/XP - upgrades between these Windows versions all worked well.
(3) Windows XP to 7 or 8 - this was the difficult one, because Microsoft disabled it (apparently due to too many problems with faulty XP installations). Thankfully there is a solution, PC Mover Upgrade Assistant, from Laplink. It's not free, but it costs much less than the Windows licence does, and as a bonus, at the same time you can make the move to 64-bit Windows (which you generally want to do these days so that you can have more than 3.3GB of RAM). So this is what I did with most of my family's computers. It worked well, but I did need to run CCleaner and clean at least the registry before starting, otherwise it may fail and not let you go back (which happened once). The risk of failure is why backing up before you start is a good idea - the disk manufacturers provide free backup software that works well.
(4) Windows 8 and the tiles - everyone hates the Tiles, apaprently including Microsoft, because they brought back the start menu in Windows 10. But there are plenty of freeware add-ons that will take you back to the old Windows desktop. The one taht's generally recommended, and which I've used, is Classic Shell, which returns you to the same desktop that we've had since Windows 95.
(5) Hard disk transplants - Hard disks don't keep working forever (eventually the mechanical bits wear out, hopefully with clicks and other noises first to warn you of impending doom in enough time) and of course as the years go by, bigger disks are both necessary and more affordable. The sockets have changed over time from IDE to SATA1, SATA2 and now SATA3 - which has become a new problem when disks fail in old hardware, because very old SATA motherboards can't speak SATA2 or 3 and the SATA3 disks can only fall back to SATA2. But apart from those limitations, hard disk transplants are easy, now I just use the free programs from the hard disk manufacturers to move everything across.
(6) Motherboard transplant - Sometimes you just want a new motherboard to get a faster CPU or because the old RAM type has become too expensive to upgrade, or perhaps because the old one doesn't support your new SATA hard disk, but sometimes motherboards do die, and for me this has usually been caused by bad (swollen) capacitors - which still happens despite Wikipedia saying it was all long ago - see this explanation of why. Once upon a time, I would just shift the hard disk to a new motherboard, install the drivers, and it worked. (It usually invalidates the Windows Activation, but Microsoft will let you re-activate if you don't do this frequently.) But eventually it was no longer good enough, when three generations of motherboard transplants caused one computer to lock up completely. The problem was conflicts from all the old hardware drivers, and I had to fix it by deleting the driver files manually from the disk (and from the dll backup store). It was painful, so now I do this the right way. After replacing the motherboard, all that you have to do is open a command prompt (right-click and run as Administrator in Windows 8), and type these commands:
This works in every version of Windows from 2000 to 8. It starts up Device Manager, where the next step is to select Show Hidden Devices from the menu, then look through for all the missing hardware, which will have its icon greyed out, and uninstall it. I've now been through several motherboard transplants on one of my PCs, doing this each time, and the solution seems to be flawless.
(7) Other hardware bits - Most of this has been straightforward. Power supplies changed to accommodate motherboards needing more 3.3V power, so that just meant a power supply swap. Otherwise, I keep old power supplies until they fail, and sometimes repair them, at least when it's the most common failure, a life-expired (noisy or stopped) fan, which is not too hard to replace. Hard disk power sockets on the power supply have changed gradually from Molex to SATA, but adaptors are easy to find. Cases haven't really changed except for the addition (long ago) of front panel USB and audio, which is so useful that I've replaced most of my really old cases that didn't have both. Keyboards and mice have had to be replaced when the old plug is no longer available on motherboards, and both have changed the plug twice (e.g. mice from serial to PS/2 then USB). Older external bus architectures (such as SCSI) used for peripherals are still available but, due to the lack of manufacturing volume, new interface cards become expensive - so I keep my old PCI cards.
(8) Some applications are a problem - Programs that have been continually updated from long ago do cause problems because older Windows versions encouraged storing configuration and data within the Program Files folder, and this is no longer writable by the user. So some programs such as web browsers have to be re-installed, as does Microsoft Office.
The old Windows 3.1 programs, which would install just fine in 95 and 98, and still work in XP, are 16 bit, which means that they won't work in 64 bit Windows 7 or 8. For that, you do finally need to use a VM (Virtual Machine) solution.
So that is my grandfather's axe - three new operating systems, five new motherboards, four new hard disks, and unlike the axe, the programs are still there working after 15 or so years.