Rebooting can be a little difficult to pin down. The first thing I would do is to see if you can get into safe mode. If the system loads in safe mode, you most liklely have a software / driver / tsr problem. If it will not load into safe mode, you have a hardware issue. Once we find where the problem lies, it becomes easier to solve.
To attempt to load safe mode, hit the F8 key when boot your system [after it detects your devices like hd, cd drive, etc. Select safe mode from the menu. It will take a while to boot into safe mode; at least 2-3 min longer than a normal boot.