User Account Control on Windows Vista is widely disliked. Many prompts of "Cancel or Allow" can be distracting, annoying, and cause significant hair loss.
The problem is, when you disabled UAC, the Security Center then starts nagging you about it being turned off. There is no way for the average user to disable the Security Center nag, since there isn't a checkbox in the Security Center to eliminate it.
There is, however, a registry key that disables this nag.
NOTE: Standard warnings about editing the registry apply. This can cause various problems if you don't know what you're doing.
Open the registry editor, and navigate to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Security Center
Create a new DWORD registry value called UACDisableNotify.
Change the value of this key to 1.
Reboot, and you will no longer get nags about Windows UAC being turned off.
Alternative Method
If you don't need to know how it works, and just want it to be fixed on your computer, download this registry file:
Windows Vista UAC Security Center No Nag
Save it to your desktop, doubleclick the file, and reboot. You should no longer receive warnings about UAC being turned off.