I work in the tech support field, and lately a great many people coming to me saying that their computer was running very slowly/giving them the BSOD/weird programs installing themselves/one case of a computer being turned into part of a botnet. All of these problems have one common cause: MegaUpload. It seems as though their server was compromised and is now sending out viruses to users who download anything from them. These attacks are affecting all operating systems, not just Windows, and in particular Macs and Windows seem to be especially vulnerable to the really nasty ones.
Use caution if you download anything from MegaUpload (I would recommend staying away from it for now), but if you really want to use MegaUpload, I would recommend having a strong anti-virus program activated.