Big white square blocks problem

Ok I've had this game for a looooong time and have seen this problem before, but, for some reason, there are giant white blocks as character's heads. I don't know what causes this problem, but my video card is a geforce 7900 gt oc bfg 256mb sli, and it's only happened like once or twice. The thing is, I know that when Guild Wars does the "repair data archive" it fixes this retarded problem but is there a way to manually get it to do that? Thanks for your help, by the way, i've reinstalled the game twice and it isnt fixing it.

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