Radeon X850-XTPE 3D tests failing-cannot run any games!

I do not use my bedroom pc much for games so I have no idea when this problem first started. I am using an Asus A7N8XE Mobo and Athlon XP3200 with 1Gb Ram and 700Gb HD's with 600Gb spare. The power supply is an 500w Antec which runs 1 dvd rom and 2 dvd dvdr's plus 3 HD and floopy.
I installed UT3 and it would not work (which is common at the moment I understand for lower spec systems till the patch is released). So I uninstalled UT3 and thought I'd play Americas Army but it would not load and said I should remove the armyops file. I then went to DX Diag and tested the 3D to find it failed on 7, 8 and 9. I have tried updating to the latest ATI drivers and even trying the older ones back to 11/06 but still 3D is not working.
Before I try removing the card and replacing it with my GForce 4 4600 spare emergency back-up I was hoping someone may have had the same problem? As I said I have not played a game on this pc for months and of course there has been many windows updates which may be a problem now?
I am concerned the X850 maybe duff because I purchased it second hand although it was supposedly unused.
Oh and another thing I had noticed since buying this card was that sometimes when starting the pc and it getting to my sign in page the whole page was covered in a white grid! Because this went away after restarting every time and it only happened now and then and seemed to disappear completely after updating the ATI driver I never gave it a second thought.
Let me know if you need any error codes etc.
Thanks

Yes I downloaded a new direct X from microsoft to no avail.
The weird thing is I have just downloaded the Nov 2006 driver from ATI once again and suddenly I get the 3D working with the Green X-Cube spinning!!! I tried Guild Wars and Americas Army and they work fine!
Now before I switched off I checked the 3D test once again and it fails! So I tried the games immediately and they still worked!
I'm stumped! Maybe a dodgy card or also a virus/bug causing the problem?
I hope someone has had the same problem?
Of course if the games continue working I will not be that bothered although it would be nice to know just in case it does it again?