Post what type of mb, what type of video card, etc. How many drives are connected to the computer; have you updated / installed chipset mb drivers?
Random reboots / freezes can be caused by a number of problems. Here are some things you can try.
Go
here and download a memory tester. Download and run the hd mgf's utility for checking the drive. If the hardware checks out ok, open your case and put a small desk fan blowing air into the case. If the problem stops, you have found overheating to be the cause.
Low quality ps cause all types of problems. Make sure you have a quality unit. Post the mgf of your ps and the wattage. Most modern computers work fine with a 350W unit as long as it is of good quality.