well my hunch is your graphics card. it has one major issue in my book <no offense still a good card thought> and that is it's ATI. Secondly.. it's a desktop/gaming card.
Also. I took the Liberty of checking into Maya 8 on Vista/7
There are refreshing bugs.
read it.. look over it.
I would recommend upgrading to the current Maya2011 sp1, if possible. That should clear up some issues. Im sure the Vista bugs for Maya 8 carry over to windows 7 as well..
**Edit** forgot to say.. the 4600 is not listed on their compatible card list with Maya2011. and the ATI 4650 card has not been tested for it, which I interpret as, not intended to be used with Maya...
so.. save that $$$ for the dumb ole PC repair shop and upgrade sir!