Merry Christmas Jephree and thanks for replying.

I was using onboard graphics chip and these games worked but not very well if you know what I mean: there was certainly no restarts just poor framerates and low-res graphics! All old drivers were uninstalled before the new card was installed. Before I re-installed XP I got a BSOD with an error code which mentioned turning of memory caching or shadowing or something along those lines. Now I don't even get a BSOD it just instantly reboots. Incidentally I have automatic restarts disabled but it still does it.
I'm not particularly worried about getting the 5200 working as I may buy a 6600GT but if its the mobo which has a problem then cash must be used for that first! I don't reckon its a heat problem as the fan is spinning no problem. It's only literally seconds or minutes after a level has loaded that it restarts itself. I'm not too sure what the advanced settings in the BIOS are (I had a look in there for memory caching but couldn't find it) but as I said it seems games that are maybe 1-2yrs old seem fine, only newer ones.

Anyway thanks again and all the best for new year!!