XP BOOT problem on IMAC

Hello!

I have a problem with my 20' I Mac
I installed a copy of Windows XP on my iMac but the copy wasnt original.
I used the windows for a month and now I have a problems with it. It starts when i turn on the computer but after 15 seconds it restarts by himself (first the colour of the desktop becomes darker and then comes out a blue screen with some text and then it restarts) The biggest problem is that the Boot Assistent is installed on the windows and i have no time to change the start up in OS. So i cant boot in OS and the Xp doesnt work. So practicly my comp is complitly out of order. Can somebody pls help me with tis issue. I read on the apple site that this problem occures when the cd from Windows is not regular and they suggest to contact their experts but I am from Macedonia and I doubt that we have that kind of experts here.

You could help me if you tell me how to get into OS from another way
if you tell me how to delete XP without Boot Assistent
and similar stuff

I will be thankfull for any kind of advise