Possible malware/virus. Possible software bug somewhere.
Download a stand-alone bootable antivirus or two. Maybe get Windows downloadable 'microsoft security essentials' run it and see if its clean. Then download Norton stand-alone, run it and see if that is clean.
Google to find this software, make sure the site you download from is Microsoft and Norton, not some clone site designed to make you download and install a trojan.
IF both anti-virus come up clean then let us know. Maybe someone can do some Safe mode debugging. Or you can make a backup of you PC to an external drive using a bootable backup (maybe ultimate boot disk, maybe something from acronis?) then do a factory recovery to a clean PC, use windows update to get fixes installed then restore the files you need from the backup.
edit do you have any friends who would prank you by putting something in your startup folder, etc? If so then talk to them before restoring your PC.