BleepingComputer.com: total system freeze, keyboard & trackpad unreponsive

I've had this total freeze problem for months, but it just happens occasionally, where the keyboard and trackpad just become totally unresponsive, task manager won't come up, CTRL+ALT+DEL does nothing, the screen is still on, just frozen, it won't even sleep when I close the lid. This is a HP dv2600 with Vista HP 32 bit SP1, all the latest drivers installed inc nvidia 185.

I've looked in the event viewer after rebooting each time it happens, but can't see anything other than reports of the last shutdown being unexpected (from me holding the power button down).

Everything else works fine, I've run Avira premium for over a year on a standard user account, so I doubt its a virus, as that says everything is clean, as does AdAware.

I've tried 'reliability and performance monitor' too, but that won't work - "Internet Explorer cannot download, the system cannot find the file specified" is the error message I get there.

So I'm a bit stumped really, any ideas?