I'm not very knowledgable about WMP. But I would highly recommend getting VLC - (click on link) _ (choose nearest mirror site) as it has in-built codecs & is generally very reliable. Try it out. You can also downld extra skins for it.

Otherwise, you can get extra codecs & also codec identifier programs, see CODECs list and alternate players for Movies which is one of the Stickies at the top of this section.

BTW I can only guess that the virus has somehow affected the association b/w WMP & the codecs. Perhaps reinstall the codecs or check your WMP options & preferences to make sure the options are set to play certain files. This may have been affected by the virus. On a deeper note, the virus might have affected the system association of programs & files toward each other. This can/should be (able to be) fixed up in one of the system control panel sections.