Computer freezing, must shut off with power switch

A 2002 Dell machine running XP.
Today after idling for a while, screen froze, nothing would help except shut power off, and wait a while.
It would not start until off for a time period.
After this happening twice I open case to find alot of dust..
Cleaned it out and it lasted longer than before, however it did freeze up again after a longer duration.
I am going to open up again to see if the computer chip fan stays running.
I'm assuming these freezes are due to over heating the processor, I've never had this problem before, or any other for that matter.
I am using this computer as a work station for programs not able to run on 7.
Any ideas or suggestions would be welcomed.
Thanks