For the system shutting down part, it sounds like a hardware issue. Vista will not cause this. (I run a Vista computer, its flawless)
UAC (Those annoying popus) can be disabled. Hold down the WindowsKey + R, and type:
MSCONFIG
Go to the Tools tab, and find "Disable UAC", and click Launch.
Then, to take care of Windows Security Center, click the popup it displays for more information, and then towards the lefthand bottom corner of the Security Center window, click "Change how Security Center Alerts Me"
As for your camera software, I dont know enough information for that...
But, like I said, degrading to XP will not solve sudden shutdown problems. That sounds like a hardware issue.