I've got a Dell Inspiron 1525 on which I've installed a 64bit version of Vista Enterprise, but now it seems to randomly (not each time) reboot when I try putting it in sleep mode or hibernate.
I've installed all the latest drivers from what I know.
The only thing I've been able to eliminate to now is that sometimes when it reboots on sleep/hibernate, then physically switching off the wireless card seems to handle the problem.
It's a Duo Core T5800 @ 2Ghz, 800Mhz FSB, 3GB DDR2 RAM.
As I'm dual booting, my 32 bit Vista doesn't have this issue.