|I would first get a free copy of memtest. You are supposed to run it for hours but any errors usually show up in a matter of minutes. Any errors...replace the ram chip.|
Then download and run the hard drives manufactures software to check the hd for errors. They too are free.
If still no joy....do a repair install as it is possible they ran an anti virus program and it corrupted system files.
I would rule out heat as the problem is at startup and the computer restarts rather than shutting down completely.
If you have a power supply tester......your in the box anyway....plug it in and let it run for ten minutes or so.
Hope this is of some help