My Win7 Pro x64 machine sometimes will begin using all memory and then becomes unuseable. I have 8GB installed, and "normal" is for 2GB or so to be used. Sometimes, I can see in my memory usage graphs (gadget) that it spikes all the way up to 8GB within 30 seconds and then constantly has activity on the HDD as if the problem is still going on. The computer is unuseable at this point and I must reboot.

It doesn't happen often or anything, but it is very annoying. When it has happened, I am using IE8, Chrome, or Firefox...it doesn't seem to matter. Sometimes it does it when no apps are running at all. 

My config:
Core2Quad Q6600, stock speed
Nvidia 780i MSI motherboard, stock speed
8GB Corsair XMS2800, stock speed
Nvidia Geforce GTX295, stock speed
Soundblaster Xfi Platinum PCI
Avermedia Duet dual ATSC/QAM PCIe
NEC PCIe USB2 controller (ATI DCT not compatible with 780i)
ATI Digital Cable Tuner with SDV box
150GB WD Raptor, 5 other hard drives, LG BD-RE drive

What's always running: Steam, Logitech KB/M tray apps, AVG