Hang-Ups

Three days ago I upgraded to Windows Vista Ultimate, and I have had hang-up
after hang-up (Not Responding) which results in a reboot.
There is no pattern to the hang-up's they happen randomly and often.
Prior to installation I checked out if my system could take the new OS No
problems were found, also my DVD/CD-RW whilst it has been found by my system
the CD etc
are not even though I have configured Autoplay.
My System...............
Pentium 4 3.42Ghz
2 Sata 300Gb Hard Drives
2 Gb DDR Ram
Nvidia 6600 V+ 512MB /DDR2 *With the latest Vista Driver*
Audigy Live 24bit Sound card
OS Window Vista Ultimate 32bit.
Your help is please ugently needed as I can't keep rebooting my computer.
Thank you