Okay, I assume a mod will move this thread to computer forum for us...
Reformatting your hard drive isn't always necessary, but if a system gets so plagued with errors or can't perform normal functions anymore, then yes - it is a great way to get a fresh start.
The first key thing is trying to make backups of your stuff. If you have a second hard drive on your system, that is the quickest way. Next quickest would be to burn things to CD-ROMs or a ZIP drive if you have one. You can also go online and e-mail files to a friend. Or store them on a server for later retrieval.
This site gives you a free 20 megs of space with no questions asked (choose "Option 3: Free Domain").
The next key thing is... GO BACK AND MAKE BACKUPS OF YOUR STUFF!

It is such a classic thing to forget something. Remember this: You are about to lose every .mp3 file, every e-mail message, every e-mail address, every internet favorite, every text file and every other piece of data you've stored on drive C.
Now - you need to have made a Windows boot disk - also called an "emergency boot disk". This is on a 3.5" disk:

Let us know if you need to know how to make one of these.
Insert the boot disk and reboot your PC - it should go to the command line. If it asks you to start with "CD ROM Support", make sure you say Yes to that option. Type in:
format C:
The computer will give you one last chance to back out and make sure you're OK with losing all the data on the selected partition, and then formatting will commence. A percentage marker will increase, showing your progress to completion.
Then you need to make sure your Windows 98/Me/XP CD-ROM is in your CD-ROM drive. Usually your CD-ROM will be known as drive d, so try typing:
D:\setup.exe
Once you find the Setup executable on the CD-ROM, Windows will automatically guide you from there.
Your first step on your new system (besides adjusting colors, wallpaper, font size, etc) is to re-install the drivers that came with any sound card, video card, printer and CD-burner you may own.
Oh yeah - and make sure you know a friend who lives close who is a geek so you can call him if anything goes wrong during these steps.