Computer Freezing

My computer keeps locking up. Everything seems to still be running, but I'm locked out. The keyboard and mouse just freeze.

At first I thought it was overheating but after cleaning it and running a diagnostic on the hardware, it clearly isn't. I ran a mem test and everything checks out there. All the drivers are working fine. I have it set to run defrag once a month, but there's been no slowing down. I barely ever use 10% of the CPU and never over 20-25% of my ram, unless I'm rendering something.
Everything is running as fast as it should, it just freezes up and there's nothing I can do but manually restart.

What could cause something like this?

I'm running Windows 7 on my main partition and I've got Ubuntu on my secondary one but I havent booted that up in months.

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Clarification Request
In addition to what ...

has already been asked for, could you make use of your Event Viewer and tell us if the System or Application logs show any problems and if so What are the Event Source and Event ID numbers for each indicated problem?

Those logs are there for a reason.

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No errors

That's the thing, there is no error in the logs. No ID, no error. Nothing is wrong as far as any diagnostic tool I know how to use can tell.

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If there are no errors ...

in the system or application logs then you have a hardware problem or MALWARE.

If you have USB devices plugged in unplug them and see what happens.

If you have a USB keyboard and mouse try a PS2 keyboard and mouse.

Try opening the case and directing a floor fan across the motherboard and graphics card. Normally heat will cause a shut down or re-boot but an overheated graphics card can sometimes result in symptoms similar to the "freeze" you describe.

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Defrag is not a cause or cure.
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I'm using a Dell XPS X8500.
Dual booting Windows 7 64 bit and Ubuntu. I do not have the problems on the Ubuntu side since it's not a hardware issue as far as I can tell.
Intel i7 processor, 3.4GHz
AMD Radeon HD 7570

The only things I've done to it are expanding the RAM from 8 to 16G and regular cleaning of the fans with dust cleaner.
The problems started before I added more RAM.

It's less than a year old.
I'm not sure whatever information would be useful; I'm not very knowledgeable in this area.

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As long as you claim it's not hardware.

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