On your computer (not this computer in question), download an antivirus and antispyware program and burn it to disc.
I recommend the following (all of these are free):
Initial Antivirus Scan:
StingerRegular Antivirus:
Free AVGAntispyware:
Spybot or
Windows DefenderAnyway, be sure to disconnect the network cable from the computer (to cut off internet) and try booting up again. Trojan viruses and spyware will try to "call home" through the internet connection, so in this scenario it's important to have the computer offline.
I'd start with a Stinger scan, then install AVG and do a full scan, then finally the antispyware scan. Once it's clean, hook it back up to the internet and reboot. Be sure to install any security updates from Microsoft via the Windows Update site.