If the computer is suspected to be infected, then:
- You can boot up a Linux based LiveCD, and scan from there. (One example, BitDefender liveCD)
- You can put the drive into a Linux based box, and use an Anti-virus there (like ESET's NOD32), to scan the drive. (Thus, nothing will get executed, no harm done.)
- In case the two above is not possible, try using Comodo's Cleaning Essentials. It's a toolbox, not an anti-virus product. It requires some knowledge to use, but it's like a swiss knife. It's very handy, really.
Last resort: Install an anti-virus that comes with a Boot-time scan. Like Avast Free. Schedule the scan, sit back and enjoy the reports (False or not).