Windows Vista locks-up frequently

[nitrocide]

Hello everyone, i'm new here. I hope you will help me with my problem.
Yesterday i've upgraded my PC anddecided to get Vista Ultimate. So I installed it without many problems. But after 10-15 minutes it froze.Completely.I had to restart it.
So it keeps happening again and again. It's impossible to work with Vista when it keeps freezing every 20 minutes. What should i do?
My PC:

AMD Athlon 64 X2 3800+ 2.01GHz
3GB of RAM
nVidia GeForce 8600GT video card

PC runs perfectly on Windows XP.

[NIGHTMARE]

its original Vista i hope pirate stuff not discuss here.Is there is any error u r getting and which anti-virus u r using