If you do have the CD for XP or 2000, just go to the recovery console and run "fixboot", and "fixmbr".
If you don't know the admin password, you can use the shareware version of saminside and slave the drive to another machine to obtain it. There's many other ways:
http://www.petri.co.il/forgot_admini...r_password.htm
If you don't have the CD, format a disk using a Windows NT/2000/XP computer, copy NTLDR, ntdetect.com, and boot.ini off the drive onto the disk, then copy those files onto the broken machines drive.
If your having this problem multiple times, and your not overclocking, you may have a weak power supply, a bad stick of ram, a flaky board, or a drive about to die <- This last one is NORMALLY NOT caused by a virus!
If you are overclocking, maybe you should loosen your memory timings, lower your clock speed, up your voltages, increase your cooling, etc.
If it happens all the time: Get a different computer repairman.
You should run chkdsk afterwards.