@P.Pilcher
What you describe sounds very like the conficker virus, often spread by usb memory sticks:
If your computer is infected with this worm, you may not experience any symptoms, or you may experience any of the following symptoms:
- Account lockout policies are being tripped.
- Automatic Updates, Background Intelligent Transfer Service (BITS), Windows Defender, and Error Reporting Services are disabled.
- Domain controllers respond slowly to client requests.
- The network is congested.
- Various security-related Web sites cannot be accessed.
You might want to check any usb sticks you have. Note just pluging an infected stick in may re-infect your system.
One safe way to read the sticks is to boot a livecd version of linux on your PC and read the sticks under linux