Need some advice please

What's causing Vista to freeze?

Here is my situation:
I'm been running Windows XP Pro for several years and just recently
installed Vista Ultima on a second HD on the same computer. When I dual-boot
into Vista, it runs just fine EXCEPT the system will occasionally freeze up
and I cannot do anything except hold in the power button until the computer
shuts down.

This problem can appear while I was moving through normal dialog boxes but
it will almost always lock-up while playing Vista solitaire or chess -
sometimes rather quickly, other times I can play for quite awhile. Seems
like the more graphic intense applications will cause the freeze up.

The only programs installed on Vista are Microsoft Office 2003. No add-in
games, etc. I've upgraded the graphic driver (Radeon X1600 card) to one that
supports Vista. The MB bios is set to default, no overclocking, etc.

I'm beginning to suspect some kind of hardware problem but I can't seem to
pin down anything. I run some high level games on the Windows XP side and
never had any problems so I don't think this is a heat related issue.

I'm seen this problem (lock-up) posted before but not seen any solutions.

Any and all suggestions greatly appreciated. Trying to determine if this is
a Vista problem or a problem with my hardware.

Jim