How do I completely reset my virus and add-ware infested slow desktop so it would be all clean?

I have a 5 year old desktop, Compaq Presario with Windows XP Home Edition. Because of the fact that we had some pretty bad virus and add-ware protection in the past, now, it slow as hell, and it's constantly showing various odd error messages. I've tried a hundred different things, downloading new anti-virus software, defragmenting the main drives, running Spybot Search %26 Destroy like a hundred times, and it has improved the situation very little. I don't have a recovery disc or anything like that for it, so I can't just wipe everything and re-install it again. My question is, is it possible for me to somehow completely reset the computer, I don't care if everything's lost, I'll back up the information, I just need to make it run faster, and MAKE IT WORK! For me the only option that seems possible now is to somehow completely wipe out everything and reset it. Can someone tell me how I would go about doing that without having to buy another Windows XP copy?

How do I completely reset my virus and add-ware infested slow desktop so it would be all clean?computer protection

There should be a recovery partition on a Compaq, unless it was deleted. On all my Compaq's it is the d: drive. There is a boot option on system start (F8 I believe). You can also run msconfig.exe to look at the boot.ini file and change the default boot device.