Hi and welcome to TSF
It seems to me that you have a bug in CMOS. Okay see the problem might be that the virus or bug has accessed your CMOS or the small batttery in your motherboard. So all the information it sends to the bios it also brings the virus into startup so even if u buy a new hard drive it would still get infected. So these are the steps you should do to reset your CMOS.
1) You must take out all cables from the back of your computer so it should have nothing.
2)Press the power botton so it will discharge PSU.
3) Take the lid of the side of your computer and look for a small battery that is silver and round.
4)There should be a switch next to it and next u should flip the switch (it only goes one way)
5) If you could not flip the switch take out the battery but if switch works leave the battery in place. If you do take out battery leave it out from 5 to 10 minutes.
6)Then reconfigure your CMOS settings when the window is displayed
7)Afterwards format your hard drive with a new installation of Windows XP to make sure that you kill all instances of the virus.
Hope this helps