Computer will turn on but before screen comes up it shuts down... then it restarts and does the same thing over and over.

You have a virus. Your system is shutting down to avoid damage to the system. You need to reload all your operating system discs and start all over.

Could be your hard drive is busy crashing.

If you can get into safe mode then do a chkdsk on c:

If not then do a Windows Repair with the same disk you used to install you OS.