Windows 7 random freeze - seems ok in safe mode

I have a Dell Vostro 3500 - picked it up 2nd hand with a clean install of Windows 7 64-bit.
I noticed it would crash randomly where the everything just locks up (mouse keyboard etc, all non-responsive) - so I thought maybe windows updates would fix it, but it appears not. When it freezes it doesn't reboot, no BSOD, no matter how long I leave it, i'm forced to hold down the power button to reboot.

After reading many threads, so far i've:

Ran Malwarebytes
Booted with all non-windows services/programs unchecked
Ran a disk check - no errors
No entries in event log at time of freeze
Left Task manager open until freeze occurred - no observations.
Checked for driver updates (not sure how thoroughly I did this though)
Ran dell diagnostic tests using F12 - no errors
sfc /scannow - all OK.

There's absolutely no rhyme or reason what makes it crash it seems - it has happened after I've started windows and not even moved the mouse yet, and also after hours of just routine use.

I was thinking (hoping not) that it might be hardware, a bad MB or HD, but it seems ok in safe mode. Does this point towards drivers?



UPDATE:


I ran SeaTools - no errors
Ran Memtest86+ for at least 7 passes - no errors.

Didn't know what to try after that so did a clean install of Windows hoping that would cure it, but still freezing randomly for no apparent reason.

Really don't know what else to try - can anyone help please?
Thanks

If anyone has any suggestions for things to try/test that would be very helpful.


***UPDATE*** Jan 2015

So I know this is an old thread now, but i'm still having the same issue. I'm no expert, but i'll just try and explain symptoms that I am seeing. Whether they are significant or not, I don't know.

Basically, since my post on here, somehow the freezes just became less frequent and only really ever froze a few minutes after I would log back into windows from 'resuming windows'. This I could work with..

However, just recently my hard drive died so I bought a new one and re-installed windows - straight away I am getting freezes very frequently again. The freezes are exactly the same as before - no reboot, no BSOD, just locks up. The hard drive light goes out and sounds like it goes to sleep, wifi light still on (even when turn wifi switch off), power light still on, tried charging my phone via usb while in this state, but no power.

I don't know what's causing this, as occasionally the freezes even happen a couple of seconds after booting up, without even getting to windows startup; sometimes after I type my password to logon and press enter...the 'bing' sound get stsuck in a loop and it's frozen. Although the majority of crashes do occur within windows.

Could this be a problem with power supply cable? I'm pretty sure it's not an over heating issue - laptop is normally quite cold, and tends not to freeze once it has been on for more than about 15 minutes.

One thing I have noticed is if I startup from a clean shutdown, it normally freezes. I am then forced to hold the power button, which prompts the 'windows did not shutdown properly...etc' message. Once I select start windows normally, it seems to be more stable by starting up this way.

I can't understand how it was more stable before the new hard drive. I kept a folder of all drivers I installed when trying to fix it before, and have basically copied what I did then for the new hard drive.

If anyone can help, that would be much appreciated