PC keeps freezing at random. No BSOD, just hard locks up.

Problem

Hello all, my PC keeps freezing at random. I could be checking out vids on Youtube, and the whole system would just lock up. Or, I could be leaving it on to download some files, and it would lock up too. It's really quite random, and I can't determine exactly when it would lock up.

As an example, suppose I'm watching Youtube, and the video's playing. When the system freezes, the audio will be stuck in an infinite looping sequence, like "R-R-R-R-R-R-R-R-R-R-R-R-R-R-R-R-R". Windows doesn't BSOD, nor does it automatically restart. I have to manually restart it. Also of note, when I reset the system, I don't get the usual "Windows has just recovered from a serious error" message that one gets when something like that happens.

-edit- Also, if this little bit of information helps, the PC sometimes can go on for several hours without hanging.

Tried solutions

Thus far I have run memtest for memory, OCCT (GPU), OCCT (CPU) and HDTune for HDD. All have come up with no errors. Then, I thought it must be software conflict, so I wrote the drive to zero and reformatted it. Surprise surprise, the system still locks up. So now I'm certain the fault lies with either the PSU or mobo. I can't really place my finger on which it is though.

Computer specs

Here are the specifications of my system, including OS:

OS: Windows XP Pro SP2
CPU: AMD Athlon II X3-455
MB: GA 970A-D3
RAM: Kingston DDR3-1333 4GB
HDD: Western Digital 500GB SATA3
GPU: Palit GTX550 Ti 1024MB DDR5
DVD: LG 24X DVD-RW SATA
PSU: Seasonic 560W X-Series

Attached to my USB ports are:

ZyXEL G-202 USB Network Adapter
Rodent

What needs to be done?

With that being said, what should my next course of action be? I'm at a loss here

A hard lock without BSOD usually points to something on the memory bus. Since you've tested your RAM, I'd suspect issues elsewhere. I've seen bad slots but on DIY builds it is far more common that something like your memory voltage isn't set right for the RAM speed in the BIOS is perhaps the most common. I've seen CPU's go bad or be under volted as well. Sometimes a BIOS update helps... If you have never updated your BIOS I'd look at the various BIOS revisions on Gigabytes website to see if any of them had a problem/fix for this issue.

I think it will be the motherboard. I have had problems with Gigabyte boards losing communication with the HDD. The normal symptom is a straight lock up with no BSOD. Another sign is the HDD being lost in the boot order in BIOS. However this problem will not always be present. I think your motherboard is suspect number one at this stage imho.

Well my computer is doing this same thing and idk what is happening so can anyone help me out, it just freezes out of no where i tried getting rid of some of the drivers which i though could be causing the problem but idk pls help