I'm using Windows XP Prof. Recently, not quite e very day, when I click on the screen a pattern of multicoloed bars momentarily fills the screen and the system reboots. There is a loud click from the speakers. This has happened only when I am in AOL but this could be because I am on AOL more than any other program. This has happened four or five times in the last week.

I would check your power supply. It sounds like you have either a supply that is barely sufficent for your hardware or it is getting ready to fail.
Your computer doesn't happen to be a Gateway product?
I ask that because a few years ago, when I was researching computer manufacturers, I noticed that Gateway put barely sufficent power supplies in their products.

I have a Systemax Venture HU2 which is about a year old.with a PSO 350 W. powersupply. I doubt that it could be failing. Is there no other possible cause for my reboot problem?

Unless it is some strange sort of virus, the other possibilities would be a video card or monitor problem or possibly the connecting cable between the two.
The multicolored bars appearing would seem to be some sort of test pattern, which would indicate that the signal from the video card has been lost.
I said power supply originally, because that is the usual cause of random reboots.

Thanks very much. I'll check what I can and hope it solves itself as computer problems often do.