My PC is Mount Vesuvius. Overheating problems.

Ok so here's what my PC has been doing:
I turn it on log in to windows, everything loads annoyingly slow, CoreTemp tells me my CPU is at 60c when idle. After about 2 mins(sometimes more sometimes less) it locks up, doesnt shut down. I've left it like that for about 10 - 15 mins to see if it'll recover but it doesnt. I'm assuming this is caused by over heating but I dont know why its so damn hot. Bios settings are at default, it isnt being overclocked. Also when a PC overheats...doesnt it normally shutdown to save itself? rather than lockup.
Oh and my household tempatures have gone down since the last time my PC was in working order because I moved to a place with better cooling.

My PC - Intel Core 2 - 2.4GHz
x2 1GB sticks of DDR ram
x2 250GB WD HDDs (one is sata the other is IDE)
x2 GeForce 7900 GTX viceo card
Antec 550w power supply
Asus p5n32 - sli se deluxe mobo
2 fans on the back, 1 front fan and one side fan.

PC history - Bought it about a year ago, had a friend build it. It ran great until
last May when the Mobo just died(sent it to Asus and got a new
one back) I rebuilt it, reapplied thermal paste and only put one
of the 2 video cards in(the other is being used by another PC)
and now it runs like crap and locks up.

Anything anyone can think of checking/changing/doing to help whip this bitch?

Also: Asus tech support is shit.

Jam it back in, in the dark.