Let me tell you a short story.
Joe bought himself a new computer in the supermarket. Joe's really happy with it because it comes with all
the new features, the newest windows and is really fancy. After some time the computer gets older and is nothing to pranck about it anymore. All sorts of problems arise: windows crashes more and more, some programs start acting strange, it starts running slower and slower... Joe hears about some new popular virus that has to be the cause of his problems. So he asks a geeky friend for advice. This friend really knows a lot of computerstuff! He solves many problems on the computer and gives advice to joe. He teaches him to use some professional looking applications to keep out viruses, spyware and some other stuff joe forgot about. Not all problems were solved by the friend though, and after some time the old problems start to arise again with some more new problems. The friend comes by and tries to help Joe with his computer. He explains to Joe that he has to do some things differently to prevent problems. All goes quite well for the next days and Joe hopes it'll stay that way. But no, some day his computer just completely crashes and he can't even start windows anymore!
The computer must be broken. Joe realises it's already very old, the new computers in the shops are at least double as fast and must work much better! He asks his friend to come by, and he tries to reinstall windows but somehow it doesn't work. That friend says "that old crap must be broken, get yourself a new computer. I just saw a good buy at the supermarket. It only costs 1000 blingbling and is way better then this old thing."
Joe goes to buy the new computer. It's advertisements look really promising!
But after some time he starts having problems... AGAIN
This is how it goes many times with computers. People don't understand it and ask advice to other people who are more techy but really don't have a clue if windows doesn't want to work anymore! But there's nothing to blame them, because it is hard to grasp any logic behind the curtains in windows.
It could well be that windows is the problem and it's not the computer hardware. So get yourself a decent operating system like linux or BSD. Linux can even run on partly broken memory sticks! (ecology howto)
The way described in the story is sad. Many old computers get thrown away which do work well or at least many parts do. This pollutes our earth. And there's a lot of money and time lost.
I'll explain you some basics to keep yourself from buying a new computer when your old one still works but you think it's broken.
First you need to know how a computer is built. It consists of several parts which are connected to the mother board. A big board with a lot of wires going to it. There is a cpu, videocard, soundcard, memory, hard drives, cdrom drives, and so on. All these different parts form the computer as a whole.
There only has to break one of the crucial parts of the computer and it will stop functioning. But most of the parts are still working, you just have to replace that one part to get your computer working again.
You could get a cheap second hand replacement found in a second hand newspaper, etc.
But did it really break? Was it not only windows which stopped functioning due to a virus? Viruses usually don't break up hardware parts at all.
Many will try reinstalling windows. But it's easier for you to check if a linux live cd boots well on the computer. A live cd is a cd which you only have to put in your computer just after powering it on and then it'll start an operating system (it doesn't touch your harddrive and is without install!). If the cd doesn't start after putting it in the cddrive you have to check if your BIOS settings are set to boot from CDROM before the harddrive. You can access your BIOS by pressing the Delete button while booting the computer.
A good livecd which I recommend is the Ultimate Boot CD (click on the link, download full version and burn it). Try running INSERT and run programs with it. If it boots up then you have a big chance your computer isn't really broken. If it doesn't, it still isn't sure if your computer is broken because INSERT doesn't work with all hardware.
There are many tools on the ultimate boot cd to check if your hardware is working or not. Analize your hardware, get to know what kind of parts you have inside with the program AIDA. Do the benchmarks, burnin tests, test your hard drives, your memory, and so on. You should be able to find out which part it is that causes the problems.
If you found out a part is broken it might be possible to repair. This is easiest with cdrom drives.
Some links to sites having information about repairing hardware:
Deinmeister
Repairfaq
Repair scratched cd's
I have succesfully repaired an old cdrom drive once, I just opened it up and looked what the problem was. I found it was the motor being stuck in a bit of dust. I pushed it a bit around to let it start and now it still works after a long time.
Repairing hardware can be many times almost impossible though. A cpu is unrepairable for example. It's way to complex and small.
Your part may be unrepairable. But you can easily replace it.
Find a good friend with good knowledge about computers. He could find you a good second hand replacement in newspapers, or anywhere. Let him replace it if you don't dare it yourself.
Some suggestions to keep your computer running a long time:
1) Put it on a cool and dry place. Don't put it besides the radiator or any source of heat. Heat is the enemy number one of a computer! You'd also want to make sure you have enough cooling in your computer so it doesn't get to hot. Which is definately a problem with the newer high speed computers.
2) Very important is to clean the computer inside for dust from time to time. There can collect some serious amounts of dust in there. That dust could make the fans work less efficient. It also creates a thermal layer so things get warmer, it can also block connections.
3) Run decent software on your computer. Software like windows which makes your computer crash can damage your hardware by crashing all too often (side note: this was written before Windows 7 came along, which is very stable, in contrast to Windows 98 and Millenium..). Try running a stable operating system like Linux or BSD. Those 2 don't have many other problems windows has like viruses, spyware, Blue Screen Of Death, and so on.