If you suspect or know your PC is infected with a virus, it’s probably wise to avoid purchasing anything using that computer until you’re sure the machine is clean. That includes additional anti-virus or security products. Chances are the malicious software on your machine includes built-in ability to steal user names, passwords and other sensitive data from infected hosts. Recently, I’ve heard from several people who used their credit or debit cards at the first sign of infection, to renew or upgrade their anti-virus protection when their existing software didn’t work or failed to update. Also, in a Live Web chat a few weeks ago, one reader described how he “stupidly” went online and bought an anti-virus product after realizing he’d infected his machine with a DNS hijacker Trojan. Consumers can be forgiven for such goofs: After all, they paid for security software, they expect (rightly or wrongly) to be