I can be on the Internet with several pages up and all of a sudden, everything closes and the PC starts rebooting.

I have Window XP Office Professional.

Any tips as to what the problem could be would be appreciated.

Thanks in advance.

You have some sort of virus that you picked up either online or somehow downloaded it. Would suggest that you run a good virus cleaner.

Here's some things to try:
Computer Restarts Itself

Hi, I agree on the heat issue. Sometimes dirt, thermal compound and I even heard of loose heatsink/fan clips a couple of times
Peace,
crigby