Help diagnosing a problem?

I am having a strange problem with my Radeon 9600XT. It seems to centre around graphical glitches and corruptions, though the symptoms are limited only to very specific things, which I'll list.

The system is running Windows XP Home SP2 dual booting with Vista Ultimate.
The problem persists with both Catalyst and also NGO drivers. Symptoms are the same with both drivers. Neither the card nor the system are overclocked in any way.


The following things work fine:

Windows XP desktop when using Office or Firefox etc. No glitches, nothing to suggest any problem with the graphics card at all.
Playing DVDs in VLC (watching them that is)
Playing AVI/DivX


The following things don't work and produce glitches:

Menus and subtitles in VLC (glitches of vertical multicoloured bars appear over them though they are still readable).
When I pause or play in VLC, the little icon in the top right has the glitches appear
During BIOS POST the lines appear, they are there all through boot with the exception of when the "Choose OS" screen appears. They disappear when I get to the login screen.
Games: Any game is so massively corrupted that I can't see what's going on. Interestingly, in Tiberium Wars the inset video clips show fine.

Running DXDiag.exe I test for DirectDraw and Direct3D, Direct Draw all is perfect, no glitches or problems. Direct 3D however there are a number of glitches, patterned alternate short green lines which are static on the display. Like so:

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Any ideas, or any more info you need?

The green lines didn't show like I intended, but basically it's an alternating pattern of about twenty lines, then a gap of the same length, then more lines, then a gap and so on.

 

Same thing would happen with my 9800 Pro. It's either the fan on the card going bad or its just really freakin old.