Windows 7 - Windows completely freezes up. What could could the problem be?

Recently my laptop started to freeze. And I mean COMPLETELY freeze. The mouse pointer disappears and windows freezes instantly in that state.
I cannot type anything, I cannot move anything, ctrl-alt-delete doesn't work and neither does anything else, I must shut down the laptop and restart.
If any sound or music is playing it freezes and the last few musical notes (or tones) become repetitive and loops ad infinitum.
The computer stays in that state like a snapshot and will not budge whatsoever unless I shutdown/start it again.
Also, it doesn't show any signs of increased CPU or RAM activity when the freezing is triggered. No visual or audible hdd turation or CPU overdrive.

I did a full virus/malware scan, nothing was found.
No errors in chkdsk
All drivers are installed and up to date, no conflicts in device manager.
Temperatures of system are normal, no overheating (verified with hwmonitor)
I also cleaned the hardware, removed dust, etc. Software-wise, I cleaned everything with Ccleaner.
My sistem is Windows 7 Ultimate 64x , 2ghz , 4 gb ram.

Apparently the laptop doesn't randomly freeze. As of yet I have observed two instances that trigger this behavior:
1. When attempting to copy files or folders from a computer connected to my wireless network (I can only view the contents of folders)
2. When I try to do a search in Emule and the search would return lots of potential results ( i.e a popular file )

The latter is really strange. When I click the search tab in emule to see its contents, after it has finished the search, the computer utterly
freezes like I just described. Same as with the copying, when I do ctrl-c with a shared file on my wireless and then ctrl-v back on my laptop.

Thank you very much for any help I may get...

Run these hardware tests

1. Memtest86 - Run for 7-8 passes - RAM - Test with Memtest86+
2. Prime95 - Run all three tests for 3-4 hours each or until fail - Hardware - Stress Test With Prime95
3. Hard drive scan usings SeaTools - SeaTools for Windows | Seagate - Both long and short tests

Ok, I will do those tests.

But meanwhile,
I've just noticed I have "VirtualBox Host-Only Ethernet Adapter" under Network Adapters, in Device Manager. I remember having configured that a while ago, but could this be part of the problem?

Can someone else please give me some other suggestions? Thank you so much.