Go here, download and create a boot CD. http://support.kaspersky.com/us/viruses/rescuedisk/
Boot from that and scan. see what it says.
Often, a nasty virus will disable the part of the AV that would detect it. So your AV seems to be reporting nothing, but in fact the virus is telling it to report nothing.
However....winlogon.exe is a perfectly normal, expected file and process. 'Some people' are often quite wrong.