Really terrible Freezing problem

I installed W7 HP 64bit the day of release and its been running fine until today. I booted up as usual and browsed the web for a few minutes before going and running some errands. When i got back i clicked the start button and then tried to load Pokerstars software. The normal UAC stuff popped up and at that same moment Windows froze completely. I couldn't move the cursor nor could ctrl+alt+del to reboot. I had no choice but turn my PC off at the power. When i rebooted i chose to load windows normally but all i got was a black screen instead of the user logon screen. The cursor was visible (in its 'loading' state) but frozen. I rebooted in to safe mode and then rebooted again normally and all was fine. I then played poker for a good hour thinking all was fine. I again left my computer for an hour so before returning and queuing up a bunch of MP3's in Winamp while i took care of some stuff in my bedroom. It played the first song find but about 15 seconds into the second song the music stopped quite abruptly. The whole thing had frozen again and i was forced to repeat the process above. I turned on my PC this morning but didn't login for an hour or so. When i did login i typed in my password and it froze on the 'welcome' screen. I really dont know what to do. This just started spontaneously happening. I haven't made any changes or anything at all. I have absolutely no idea how to troubleshoot this apart from running some virus/malware scans although i'm pretty diligent and sensible about that so i'd be surprised if that was cause. I've read in other threads about disabling hibernation and sleep but i'd already done that some time ago. If anyone can help i'd greatly appreciate it. [Edit] Sorry about the giant block of text. It wont seem to save my paragraphs.

I did a system restore from 2 days ago and I got to the logon screen so i figured it was ok now. But this time it actually froze whilst i was typing in my password! I ran Windows Memory Diagnostics Tool and i sat through it and it reported no problems but i couldn't actually see the results because when it rebooted i couldn't login.

IggyAZ said:

Have you tried booting off your Win7 DVD and try the repair function from the menu?

I'll give it a go. [Edit] There's something very seriously wrong. I tried booting from my W7 CD but even that screws up. I did the whole 'Windows is loading files' bit, then 'Starting Windows' and then i got to the blue screen with wavy lines and leaf in the corner. I had a mouse cursor that wasn't frozen but nothing at all happened. I waited for 3 or 4 minutes for it to do something but it never did. So i cant login normally and i cant even try to reinstall windows by booting from the CD. All i can do is login in safe mode.

I've had something approaching success. I decided to remove my sound card (the last bit of hardware i'd installed, but still some 2 weeks before this freezing problem happened) as well as moving both sticks of RAM to the other slots on motherboard. I've successfully logged into Windows for the first time today (apart from safe mode). I've no idea if a sound card could cause that kind of disruption or if it must have been the slots the ram was in. I'm going to leave my PC idle for a good while and then return to see if i can replicate the freeze.