Its very unlikely to be a psu problem, if it can get into windows in safe mode and do stuff without crashing, it will be using basically no more power than with the drivers installed, also pre turning on a game/loading it in any way the peak power usage of a system is during start up, not getting into windows where power use is significantly down compared to the first 10-20seconds where everything starts up in full power mode and even things like fans use their highest amp ratings to get moving.
IE if it boots, its highly unlikely to be a psu issue if it then crashes at IDLE in windows.
Assuming its a fairly standard windows install(Fresh install?) fully updated and ready to go, is it crashing without the driver installed? What about opening a video without the driver installed and no hardware acceleration coming into play.
When you stick a higher power device in an PC, it can change things like a CPU that was stable at 4Ghz, under load with an extra 100W of gpu being used, that stable level might come down to only 3.95Ghz, or maybe 3.5Ghz because the 4Ghz was borderline stable in the first place.
I also just noticed you're on Win XP, are you 100% sure you're choosing the right driver(I honestly can't remember if win XP will throw a fit if it thinks its a driver for a different OS). Is it the 64bit XP, did you try a much much older driver, maybe do a google search see if you can come up with the most stable driver for Win XP, which might be completely different to the lastest most stable driver for Win 7 as most companies for higher end stuff are focusing on drivers for Vista/7 over XP.
Its definately possible to be the PSU, but frankly I'd expect non starting and for it to crash without drivers installed and in safe mode aswell.
Checking various bits in alternate PC's is often the best way to check if hardware is faulty or not, if the card crashes in any computer its in with the drivers installed, its likely a duff card, I'd try and check with the latest drivers on a win 7/vista computer if you can arrange that because that would also help to rule out XP problems.
I keep seeing this VBIOS issue discussed, and I've seen very little real evidence it exists, Rroff thinks Asus aren't always great with SLi, and thats about it, though dozens of the best overclockers in the world haven't had issues with Asus and SLI and I've seen very little evidence that Fermi has more of an issue. Its more people trying to diagnose a fault, coming up short and guessing at various other potential problems, which isn't uncommon, enough people guess the same thing enough times and suddenly it becomes "internet knowledge" and therefore proven
