GeForce 8800 GTS OC 320MB GDDR3 HDTV/Dual DVI problem

ok I have just brought a new computer which consists of:
http://www.overclockers.co.uk/show [...] =BU-043-OK
also using windows Vista
now i have also brought this geforce 8800 gts card aswell. well i have had no problems really up until now. i play oblivion, i have been playing for a few days and it works perfect on the highest settings i WAS very impressed. now i have got to a certain part where there is a lot of grass around me. the game then starts to judder a bit then freezes then , the screen goes black and the computer restarts itself. i have the newest nvidia driver but it makes no difference. the other problem was i went back to 1 of my favourite games empire at war. with the highest setting on this game worked but wen close up on tatooine levels the dust and smoke makes my computer start to slow. is there a prob with the graphics card of the drivers or it it something to do with vista?

to be honest im not a computer wiz but i have read some people are having the same sort of issues with other games. i mean oblivion works wicked, but wen u get to certain part of oblivion it does that strange crash thing, to be honest i dont think it happens wen u take away the grass detail to an absolute minimum. i think its a software issue with the crappy drivers available.