List exactly what happens when you turn on the PC step by step, any error messages that come up, etc... From how you described your issues it can be anything from a virus/spyware to failing hardware.
You'll probably be better off re-installing Windows at this point, you may have system recovery disks that came with your system, just follow the instructions on them or in your manual. Some systems also have a restore partition on the hard drive, you can access that though a key when the system boots up. Note you will loose all of your saved files and settings after you do that.
What happens when you try to logon to your regular account? You have not given any details yet. If you are getting miscolloered screens, and the computer locks up, could be a hardware issue with the video card also.