If the virus definitions are old on your virus scanner, the virus won't be detected. Some viruses, like Klez, are smart enough to disable your virus protection and will prevent you from re-installing it until the virus is inocculated.
Could be the I Love You virus (or a variant):
LoveLetter arrives as an email attachment named: LOVE-LETTER-FOR-YOU.TXT.VBS though new variants have different names including Very Funny.vbs, virus_warning.jpg.vbs, and protect.vbs. The subject of the message containing the attachment varies as well. Opening the attachment infects your machine. This attachment will most likely come from someone you know. Don't open any attachments unless you are sure that it is virus free. If you're unsure, ask for the sender to confirm that the attachment was intended for you. You'll know you have the worm if you have difficulty opening MP3 and JPG files.