It's very likely that the problem is your graphics card.
I had a similiar issue on my old pc when i upgraded the graphics card. i went for an GeForce 8 at first but that was taking too much power and causing hangs all the time just to boot, or maybe just after logging in if i was lucky. replaced that with 7600 now, whcih i think requires less power and doesn't hang at all now, except for those caused by memory leaks in CoX. I dont know about WoW, but Guildwars overall seems to be a much more polished game in terms of graphics performance and game stability than CoX so that may explain why you never have problems there, although that doesn't help unfortunately. Its wierd that it happens even when your pc isn't in a game, again, i had that too occassionaly, but replacing the card with a lower spec one sorted that. My old pc spec is an intel P4 1.9mhz 1GB RAM GeForce7600 with a 350mhz PSU. The hangs with the old graphics card started to cause corruptions in sys files so after replacing the card i reinstalled the OS as well just to be certain i was starting from as clean a slate as possible. You are using a lower spec card than the 7600 so i would not have thought power drain would be an issue as it seemed to be for me. Just in case though, if you do have any extra cards in your slots that are not strictly necessary it may be worth removing them from the picture to see if less power drain on your PSU helps, and if not then that should rule out that as an issue.
sorry, i cant be more specific, i've sort of brain dumped what i did and thought at the time i had a similiar issue. Good Luck.
been lot of agp problems lately it seems
After much experimentation including a spare graphics card it turns out the PSU was at fault. 
Wish I'd found that out before I reinstalled windows.
I tied a noose around... wait tech support.
Have a basic PC (Amd 2500+ / 5900 FX (AGP) / 1Gb RAM / XP pro SP2), It normally runs a single CoH client with zero issue. Lately though it seems to hang for no obvious reason.
Checked:
1)Heat - three fans in the case and according to Motherboard Monitor the case ony gets up to 40c on very hot days or when sun is shining on PC (its in a window). But hang happens at times like now when sun isn't even up...
2)Drivers - I have the latest 175.16 Nvidia drivers, they work fine on my main PC (considerably better PC), and generally they seem to work fine.
3)Software - PC is running XP Pro SP2 (can't use SP3 as some of the drivers I need won't work) critical updates are done but most of the rest aren't. No sound installed (media device is disabled in bios). Software running in .net 1, 2, 3.5 (Hero Stats is occasionally used) Octopus (key rebroadcaster) and my keyboard drivers (Nostromo n52). All of which runs on my main PC (and laptop).
What happens is simple my PC hangs, no input excepted and only reset switch works. Occasionally it happens while playing but more often happens when I get to the character select screen. The textures on the character doesn't work properly and body parts are missing. But this doesn't happen all the time. Occasionally it aslo hangs with any games loaded when its sitting at the desk top.
So could it be that my graphics card is dying? It works fine for other games (WoW & GW) and sometimes the hang is when there is nothing intensive happening. Or could it be the PSU?
Or something else?