Cannot repair/restore OS

Hello,

I have an HP Pavilion notebook with Windows XP.
Unfortunately, my laptop was infected with a virus or something problematic. I tried using System Restore to see if it would help any, but I could not advance far enough to accomplish anything. (Clicking 'next' did nothing).
I downloaded an antivirus, and it deleted the infected files. However, now I cannot log into my laptop. After entering my password, I am almost immediately logged off. Even when I use safe mode, the same thing happens. I have tried booting from a disc, but my laptop turns itself off before it can repair or reinstall the operating system.

I am not very good with computers, and I am not sure what to do now.

Any advice or tips would be greatly appreciated.

Thanks!