It technically could be a virus that got itself lodged in the boot sector of your hard drive.

Other than that, I would either blame your video card (possibly due to dying hardware or heat problems), or possibly faulty memory. (could be the CPU/mobo, but unlikely)

If you are still encountering problems, after updating your antivirus software and all, try this:

If you have a CD-RW drive, download memtest86 here:
http://www.memtest86.com/memtest86-3.2.iso.zip

After downloading the ISO, burn it onto a CD, then boot your computer from the CD. It will run a comprehensive check of your computer's RAM and report any hardware errors. If it finds any memory problems, then consider replacing your memory as it's likely the cause of unstable system operation and strange anomalies.