PC Freezing

Hello everyone,

Before I start describing my problem I just wanted to say that I used the "search" function bug got lazy and impatient in the process of going through all the threads.

My computer freezes!!

How: I turn on PC, browse around for a few, VGA fan goes crazy and everything freezes immediately. After a couple seconds it goes all 1 unique color, black, grey, brown, it just turns into 1 particular color with too much brightness. All I can do is reboot.

When: After I turn on my PC.

How do I overcome it: I don't, after several reboots I can just get lucky and it won't happen again. That's why I barely restart my PC, I leave it on most of the time.

I don't think it could be overheating but maybe some particular issue with my Windows 7, can't pretty much make a guess there..

Anyway, I'd be greatly appreciative if anyone could help me out on this one.
If I had the funds I would just buy another PC, I love computers and mine is getting a bit old, although my VGA is ATi Sapphire 4850 HD, the motherboard is a bit noobish.. If you need details on my hardware just ask me..

Thank you in advance.