I thought I'd relay this information over here to Community Discussion since this is a very serious issue and needs to reach as many as possible.
Recently one of the Nexus staff members had their account compromised, and the hackers managed to replace downloads for several of the most popular mods for Fallout and Elder Scrolls games with files containing malicious software.
More information on the mods affected in this news post: http://www.nexusmods.com/skyrim/news/12350/?
If you downloaded any of those files while it was happening, then get rid of them and do a thorough check of your computer!