PC Freezes Only On Youtube

Hi!
Almost a week I have this really annoying problem.
I use firefox and every time I'm trying to watch a video on youtube seconds later while the video is playing, the image freezes, the sound continuines to play for a second and then the cursor becomes bigger/smaller and there's an awkward sound like ''grrrrrrrrrrrrrrrrrrrrr''
I've searched the Internet for solution and I did everything.
I've reinstalled firefox and flash player, updated every single driver, disabled the antivirus, cleaned the registry, cleaned the cookies and the history, checked the temperature (normal values), checked the event log but nothing.

Note: The day that I've noticed that crash on youtube I installed the latest graphics card driver and the Firefox 4.01 Beta.

OS Windows 7 Ultimate 64-Bit CPU Intel Core 2 Duo E8400 3.00GHz Motherboard Asus P5N-D Memory 4 GB ( 2x2GB ) Graphics Card Ati Radeon HD 4800 Series 512MB

I will try this.
And no I didnt test if this is happenning to another browser.
I will try the system restore first.
Thanks a lot for your instant reply:)!

Make sure that you have the current version of Flash Player. Go to filehippo.com to get download. Be sure to get the one for non-IE which works for Firefox.

Problem solved with a simple system restore!!!!!!!!!!!
@ oneextraid : I had the latest flash player.
What would cause such a weird problem?
I have to think excactly what I did.
If I will find out I will post it for a warning to you.
Anyway thank you very much :).

Update :
Firefox keeps freezing but the Internet Explorer is OK!
It seems that the betas ruined my firefox for good.

Help Please!!!!! The problem is back with every browser!!!!!!