You could have a virus or the video card is starting to fail because of heat problems. I have also seen the same thing happen if the drivers for the card are not up to date and the resolution is high on the monitor. I have also seen this happen with too many startup items going and not enough memory. You indicated the only thing you see is explorer running, have you scanned for viruses or do you have an up to date version of antivirus product. Is it setup to scan incoming infomation from the net or for program installs. I ask this because my virus software just notified me a piece of email was contaminated and I read it differently to see it was an official looking piece of mail giving an update to explorer but not from Microsoft. In the attachment was the virus. Between Microsoft, Norton and McAfree, I get notifications once or twice a day of new ways to make our lives miserable on a computer.
On the video if the card doesn't support the frequency which you are running at or resolution it overheats and refreshing every so often happens as the card tries to stabilze the system similiar to a reboot. Sometimes the drivers are the cause, sometimes the memory on the card. Good luck
