Computer Keeps Rebooting

Since last two days whenever I start my computer it keeps rebooting. I have windows XP Professional running on it. It had Windows 98 before and then was upgraded to Windows Millenium whcih was upgraded to WINDOWS XP Professional. I tried to start itin safe mode but it won’t startin safe mode either. I tried every possible solution to make it work but didn’t work. Please tell me how to make it work i have some important data on the HD which needs to be recovered.