Windows XP restarting suddenly. Any help is appreciated.?

Hello everyone.

I've had my desktop computer for almost a year now and in the beginning, I installed Windows XP x64, Ubuntu 10.04 and Windows 7 32 bit all on the same hard disk and have never re-installed any one of them yet. Everything was working fine but a few months back, the Windows XP operating system began to restart suddenly when I reached the login screen or just after logging in to Windows. This only happened occasionally and when the computer didn't restart, Windows worked fine after that, with no slowness or any other effects. And this only used to happen with Windows XP, never with Windows 7 or Ubuntu and so i figured it was most likely to be caused by some malicious virus or a driver problem in Windows XP. I checked the computer with Avira Internet Security and checked all drivers and updated them but it still happened. I was fine with it for a while because it didn't use to happen that often. But now, it is happening much more frequently. It has happened 4 times in the last 2 days (3 times consecutively) and unlike before doesn't restart suddenly, but instead gives me a blue screen . So now I want to try entering Safe Mode(to attempt a System Restore and such) but I'm not sure if I can since I enter XP using the Windows 7 boot screen. So my first question is, Is there a way to enter Windows XP safe mode from the windows 7 boot screen?. And secondly, Do you have any suggestions concerning how to solve this problem? I know this is a common question but I find it strange that it only occurs in Windows XP. And it didn't use to occur so frequently. I haven't installed any new programs or drivers lately and I have checked the system for viruses many times but found nothing.

Finally, sorry for the long narration and thank you in advance :)