Make sure that if you are uing OEM recovery disk you have as close a specification as you can to when you bought the system. that means take out the graphics card that you might have added later & the USB priter/DSl modem etc..Now run a recovery..
Here are the steps:
0) Take out all the extra H/W that your computer did not come with, Printers, USB peripherals, etc.
1) Format & recover the system. To format, either use the windows CD that came with the computer (boot with the CD in the drive, it should be picked up by default) or when booting when you see your computer logo, start tapping F10 This should take you to a recovery menu. If you have updated your system regularly & have the latest Service Packs, you may get a message like the version of windows on the system is newer than what you are trying to install. Go ahead & install anyways.
here is a site that shows you step by step what to expect: http://www.windowsxphome.windowsreinstall.com/installxpcdoldhdd/emulation.htm
2) Install a good AV BEFORE you plug internet in. (I use Avira & Spybot S&D)
3) Update AV the FIRST thing when you go online.
4) Enable windows Updates so that your computer has all the available patches. In fact I'd say after updating AV, go to windowsupdates.Microsoft.com & get ALL the updates.
5) Now get a good Firewall if you have a always on internet connection.
6) Get some good Anti Spyware/Malware too, Spybot "Search & Destroy" is what I use, its good & it come free. Also get something like McAfee's Siteadvisor, which warns you which sites are known sites for sending spyware/malware/spammers.
