heres 2 ways u can try clearing the HD
1. pull out the HD and shove it to a different system, set the jumpers to slave, go to My computer right click the secondary HD and click format
2. if ur planning to reinstall windows, then boot from ur Windows xp Cd and during the setup u can formatt it from there.
there are other ways to format HD such as software but these are the basic ones
90% of the time u can wipe out viruses,etc by formatting the other 10% can be permanent