[quote]
If you have a manual for that computer, this would be a good time to take it out and check if there is anything written about “how to restore Windows”. [/quote]
Basically, manuals that comes with your PC doesn’t have info about your Operating System. It would only have instructions about the Hardware or maybe a vague description on how to install an operating system. Manuals for installation of OS’s comes with the CD, in which he doesn’t have.
Anyway… what kind of virus infected your PC?
It would help if you give the name of the Virus, or be a bit specific on how your PC won’t work…
Can you still get in your desktop?
if not, try getting a boot disk
bootdisk.com/bootdisk.htm
Now after getting a boot disk. Start your PC and restore your system registry to a time when you haven’t been infected yet.
Try this on the command line
SCANREG /RESTORE
If you can't seem to figure it out, try
scanreg /?
The command would let you choose a backup in your PC of your choice.
Now after that get online and go to
www.housecall.trendmicro.com and check out what kind of virus you got and removal methods. Or just run your antivirus software if you have one. Don’t waste $$$ on your PC while you can still fix it without the setup disk.
If you can still get into your Desktop, then just go to the website i mentioned earlier. Also restore your system files as i mentioned above.
Or you can stop the Virus from running everytime you start your pc.
Step 1:
Look at your startup folder
Default directory for WIN XP
C:\Documents and Settings\All Users\Start Menu\Programs
Look for programs that have been added there which are not supposed to run. (I assume you know which programs you use right.)
Someone who makes a virus would not be stupid enough to leave it on the startup folder but look at it anyway. You’ll never know!
Step 2:
After checking your startup folders click the
start button->run->
Then type msconfig
Look for programs that runs when your computer starts and remove anything you don’t need. Just write down what you remove so you could restore it if you make a mistake 
By the way you should look at the start up programs and win.ini
optionally on autoexec.bat
Step 3:
After step 2, click the
start button -> run-> regedit
Now these are the most common places a writer would drop his Virus or worm
HKLM\Software\Microsoft\Windows\CurrentVersion\Run
HKLM\Software\Microsoft\Windows\CurrentVersion\RunOnce
HKLM\Software\Microsoft\Windows\CurrentVersion\RunServices
HKLM\Software\Microsoft\Windows\CurrentVersion\RunServicesOnce
HKCU\Software\Microsoft\Windows\CurrentVersion\Run
HKCU\Software\Microsoft\Windows\CurrentVersion\RunOnce
Remove any items which looks suspicious.
Note if you don’t want to use step 2, you can also open these files
win.ini
autoexec.bat
Well there are other ways to load up a virus into your PC, but if you got your Virus from IE exploits which could get you infected without knowing it,(Mostly from embedded scripts that runs when you open your email) then the three steps above are enough to pinpoint the location of the Virus.
But if you got a virus by downloading a program , then thats a different story.
Note: Before i forget, if you are really infected by a virus, then do not just delete the executable file if you find it. (Some people think thats enough) PC Viruses is called a virus for a reason. Like biological ones, they replicate. Which means it could copy itself or make a backup copy somewhere in your PC and run with other programs. The steps i mentioned are a few ways of detecting them. But knowing this is not enough. You should get an Antivirus software if you want to play it safe.
Oh!! Another thing, you can go to the store where you bought it. They are supposed to be required to give you a CD of whatever OS you got installed when you bought it. Most of the retailers are required to give a CD of an OS of your choice if you buy the whole package from them. But if you just bought certain parts like only the mother board or HD etc. etc. then hehe, too bad for you 
Anyway i hope this post helped 