In addition enter msconfig>Startup and uncheck Picasa first before uninstalling with Revo so that that particular Registry key cannot remain because it is running at the time. Startup Programs - Change
Then do the same in msconfig>Services after Hiding All MS Services. You really only need you AV running at Startup for best performance, and it should be free lightweight Microsoft Security Essentials.
Reboot, check to make sure it remains turned off in both places, then uninstall with Revo in Advanced mode. Revo Uninstaller - How toTo Completely Uninstall Programs