It depends. Darroll is a bit confused since the viruses won't actually be active since you won't be RUNNING off that drive. To change things around a bit from what darroll said, it would depend on how good your A/V program is. Some will only catch viruses that are currently running in memory (i.e. running the drive the viruses are on). I would try Avast, MBAM, SpyBot S&D, and Ad-Aware...in that order.