could it be xp playing up?

If you want my opinion, then NO

In your opinion what could it be?

ermmm..... that would be negatory in answer to your question as\to wether it would be an XP problem or not.

block glitches are, as I say or assume, a hardware glitch - the reason I say this is I have had a similiar problem. Not to say I'm correct btw.

And what did you do to get rid of this problem.

OK.

2 occasions where this happened.....

1. My card was overclocked. Also overclocking the PC will affect the graphics card as upping the FSB will take the whole of the Northbridge side of the Processor with it and the associated AGP Bus is very sensitive to O/C'ing.

AGP Bus=44Mhz O/C'ing this by more than 10% will cause problems on most system.

PCI Bus=33Mhz O/C'ing this is more tolerant, but like anything it has its limits.

2. My card was overheating. AGP cards are poorly designed to usually fit upside down, causing the often fitted ball bearing races to run off their bearings, causing overheating. I only just managed to catch a 300+ quid graphics card before melting as I checked the fan.... only to find it wasn't spinning.... 'nuff to say I shut my PC down immediately.... but was V lucky.

I re-greased the bearings and other stuff and no more glitchy/blocky stuff

addendum:

....as upping the FSB will take the whole of the Northbridge side of the system with it.....

HTH

So how can i tell if it is overclocked?