Desktop or laptop (I can't tell)?
Any indications of overheating, have you checked that the fans are working as they should...check the CPU temperature...checked the system for dust, etc. that may result in overheating?
Have you tried using XP's System Restore to return to a situation where problems did not exist?
Can you boot into safe mode? If so, do the same symptoms manifest themselves?
Have any trouble connecting to the Internet...updating your AV program or accessing Windows Update?
How to Avoid Overheating - http://www.hardwaresecrets.com/article/142
Cleaning the Interior of your PC - http://www.bleepingcomputer.com/tutorials/cleaning-the-inside-of-your-pc/
Have you checked Event Viewer for possible pertinent errors noted?
How To Use Event Viewer - http://www.bleepingcomputer.com/forums/topic40108.html
. I would look at the last five errors noted on any tab, hopefully that approximate your freezes.
And...throughout these problems, you get no onscreen error messages?