HTPC shuts down at 69C

I have been trying to build myself an HTPC based around this case:

http://www.silverstonetek.com/products-lc02.htm

I have an Asus socket A motherboard, sempron processor, 512mb ram fitted along with a 200Gb hdd and slim DVDRW drive. I have installed XP pro. If I leave the machine on it shuts down after about 30 mins normal use. No warning messages just a slight "clunk" and it's dead. It won't reboot until I remove the power chord and put it back in. This has made me think it is either the PSU or the CPU overheating.

I have installed ASUS probe which records the CPU temperature and basically the machine seems to turn off at around 69C. This is under the default threshhold though so i'm not sure if it's that. If I run a CPU eating exe on startup that drives the CPU at 100% the temperature climbs much faster to 69C and the machine dies so that makes me think it's that. When I place my hand on the heatsink it is very hot, but the PSU is boiling so that makes me think it might be that.

Am I right in thinking this is something to do with overheating? Any ideas?

All help VERY welcome

Cheers

That sounds like overheating, yes. Small cases like that can be difficult to cool, be sure you have all the fans that will fit and that there is space around the case for airflow. Check that the heatsink is making good contact with the cpu, and that the thermal paste is good quality (arctic silver is the best) and fully connects the cpu and heatsink. Check that the fans in the psu and on the cpu are running. Try leaving the case open and seeing if that helps.