Upon startup, my conputer keeps rebooting itself automatically. Can't get past this. Help!

Most common problem is the restart button is stuck in, make sure it is free and jammed in at an angle.

If this is happening after it boots in could be that your power supply is probably going south.

If this happens while using PC at random times expect an overheat condition. Check all fans for operation and cleanliness (case fans, processor fan, power supply fan, vid card fan if applicable).