I was unable to temporarily stop AVG antivirus in Vista until I saw this tip from highmastodon:

1. Shut down AVG Control Center by right-clicking the AVG icon in your taskbar & choosing Quit.
2. Now click the Vista start button and type "AVG" in the search box. AVG Control Center should appear.
3. Right-click AVG Control Center and choose "Run as Administrator"
4. Now you can open up AVG Control Center, double-click Resident Shield, uncheck "Turn On Resident Shield", and save the changes.