It's a virus that's lodged in your Windows/System32/ directory. These viruses specifically target system files and functions. Next time you start your PC, shut off the internet and run a full system scan, especially your C drive, where the system files are normally located. It'll reveal a H-key virus or a high level trojan. Clean it out and you should be fine. Make sure your virus definitions are up to date, before the scan. This isn't a big deal, don't worry.