Video Problems

Hey everybody,

My computer has a major problem. At first the screen used to freeze
randomly and for no apparent reason and sometimes it would just go
black after freezing for like 5secs.

When that happened i had to restart my computer, but i used to get
the same problem, so i had to wait a while (like 1-2hours) before i
could use it normally again.

Now my computer won't even turn on! The fans go on and
everything, but the screen stays black and i don't get the beep of
when the hard drive has loaded. My screen stays black and just does
nothing. I havent gotten any new hardware or changed any settings.
It just started one day out of nowhere.

And if this is any help i got a few BSOD saying something about my
video card and once at the first boot screen somethimg about "com
80". I tried a different video card but i still get nothing, please help.
Thanks in advance...