Solved: Having a snow storm!

I bought a 19" Acer LCD monitor last week and wasn't happy with it so returned it and swapped it for an HP w1907v. The Acer wouldn't set the desktop properly and was covered in snow. The HP is set up but the snow is still there, white normally, but red for 'hungry' applications. I didn't notice the snow on my old 15" monitor, but as I have some recently developed vision problems I probably wouldn't have seen it on a small screen.
I've removed, cleaned, greased and reseated the graphics card, checked all cables and connections. Also plugged monitor into a separate socket, not the surge protector. I moved my cordless phone away and also tried temporarily removing my wi-fi. All to no avail.
The GPU is 57c at idle and tops out at about 65c when worked hard. All case fans (4) clean and working. No cables obstructing air flow.
No hardware is more than 12 months old.
Any help greatly appreciated. Thanks in advance.
Specs:
AsRock 939Dual Sata2
AMD Athlon 64 3600+
2048mb DDR PC3200
Nvidia 7300GS 512mb PCI-E
HP w1907v monitor on vga cable (no DVI output on monitor)
450w PSU rated 25A at 12v
Windows XP Home SP2

 

Have you tried the PC in another wall socket? Try moving the whole thing to another location in the house.

 

Yes I have tried another socket. I am going to be moving things around soon. My son is bringing over another graphics card to try out. It might be the problem. Thanks Lefty.

 

Have you tried reinstalling your video drivers? Driver corruption isn't just a myth, it does happen.

 

Yes I have the latest ones and have tried the two earlier ones as well. Thanks Skivvywaver.
Edit: I've just re-installed all three versions in turn with no difference. I did notice that the snow appears with just the 'bog-standard' MS VGA driver and also appears on the black and white set up screen when booting. I think that when my son tries his card tomorrow (it's a 8500GT) we'll strip the case out and refit everything to see what difference, if any, that makes. Thanks for the help so far.

 

It was the graphics card at fault. New card (8500gt with 512mb) is crystal clear.
Thanks again.

 

That was about all that was left! Thanks for following up.