Additional Tasks to Isolate a Problem Driver
I doubt the memory itself is the problem. I'd still suspect a device driver or other resident program has a bug, but it's painful to try and isolate which one. I'd guess you have at least 20-50 programs running in the background, in which one of them is the culprit. The difficulty is some of these are critical for normal operation. For example, you can't easily run without the display driver, since you'll no longer be able to see anything!
Here's the process I'd use. Select Start, and right-click on Computer, and select Properties. Then select Device Manager (upper-left on Windows 7 and Vista).
Go through and disable as many drivers as you can live without temporarily. This means you'll need to keep the computer, disk drives, IDE controllers, Keyboard, mice, monitors, network adapters and likely some of the others. If you see a driver you can live without for a day (for examples: audio, imagine devices, maybe USB if you're keyboard and mouse are not via USB, printer, etc.)
To disable a device, expand a selection, and click on a specific device. Select the Driver tab and click on Disable. If after a day, the problem does not occur, you'll know it's one of the drivers you disabled. You can then go through and enable a few at a time until the problems reoccur. As you can see, it's not quick or simple, but it has a good chance of isolating the problem.
Typically the odd-ball, rarely used drivers are less tested and have a higher likelihood of problems.
If the problems persist, it could be one of the drivers that you couldn't easily disable. For these, the display driver is one of the largest and complex drivers, and is more prone to design errors. If you're using an off-brand (i.e. not Intel, Nivida or ATI), the display driver would be my next choice to try. You indicated you already have the latest drivers. To check the display driver, it's necessary to install a different brand video card with a different driver. It would be ideal if you have another computer you can borrow the display card/drivers for this. Again if the replacement causes the problem to go away, you've found the cause.
We'd love to hear your results - positive or negative.