Wow woke up this morning to see all these replies! Erm, thanks

OK:
Descenterace - Could be some kind of build up I suppose, but I'm not sure I can clean it out
GodisanAtheist - OK I'll try that too (
mat_dxlevel 70 
)
Stratcomm - Warranty also says 'Customer undertakes to carry out routine day-to-day preventative maintenance
as may be recommended under the Customer operating instructions supplied with this equipment. Now in the guide it says to avoid putting the computer in dusty areas or near sources of heat, and I haven't. The climate here is quite cool at the moment aswell. It also gives instructions for other maintenance such as calibrating the battery, but nothing about opening the case, so I don't think it qualifies as 'day-today' preventative maintenance.
Now my main problem with calling tech support is I'm not the buyer of this laptop, Dad bought it, and I don't live with him. I doubt their going to be willing to deal with his 16-year-old son! So I have to wait till I'm there, but I'm going on Friday and over the weekend, so I should be able to sort all this out then. I'm taking some boxes

Short of impersonating him in an email (which I doubt he'll agree to!) my hand are tied at the moment. My stepdad (who is quite an experienced techie) also advises against opening the case without their instruction.
If it was just throttling they would probably have a case, at least, but half the time it doesn't throttle, it just powers off!
Evesham are actually quite a major mail-order PC builder over here, this laptop is basically a rebadged MSI chassis that they've put the components in.
WMCoolmon - As a general indicator of game performance, I never go below 25fps (40-50 is typical) in Half-Life 2 at 1280x800, all details high except AA, AF and reflections. CS:S stress test comes out at 79fps. FS2 SCP it's unusual for me to go below 50. So yeah it is quite capable of playing the latest games, if it weren't for this problem.
Descenterace - Why put an X700 in a non-gaming machine for marketing purposes when the only real advantage to having one is gaming?

Nuke- The bottom has the serial numbers and nothing else (no warranty void seal). It may be easy to reach the cooler, but I still don't want to risk it.
Thanks for all the advice everyone so far! I'll get back to you all once I've tested games at low details...