Need help

I have had some problems with my computer. Sometimes when i turn on my pc, my picture is messed up with funny colours and some lines. I can still see ,my desktop etc. but its still very annoying and requires me to shut down my pc and turn it on again, in order for it to disappear. I really dont know what the problem could be, so i would be very gratefull if someone could help me out on this one. I am not sure if it is my gfx card there is something wrong with or my cpu.

My pc:
p4 3.2 ghz
Ati Radeon 9800 pro 128mb
Asus p4p800 deluxe

 

Could be either one. Trial and error.