Dell Optiplex 160L Motherboard

Question

Hi my optiplex 160L would restart on its own. After talking to dell and from changing power supply, hard drive, and reinstalling windows xp prof. The computer would still restart on its own. I was told that the only thing left to do is to change the motherboard. 1) How hard is it to change the optiplex 160l motherboard? 2) Do you have any other sujestions 3) Do you know where I could purchase optiplex 160L 4)What things I should look for when purchasing motherboard.Thank you for your help