AVG Gone MAD Again?

While doing remote support for our client to removal malware, I got a very weird problem. I could not run any programs except Windows Explorer. Whenever I tried it, I got an error message saying: “Windows cannot access the specified device path or file. You may not have appropriate permissions”. However, I was able to start any programs normally if I chose “Run as…” as another administrator. I first checked all processes, services and loaded dll modules for malware. All of them were clean. I then checked the .Exe file type settings and file association. There was no exe hijack. I also checked the security permissions on one sample program and reset them even they were fine. But the problem was still there. No much useful information could be found using Google. At last, I noticed that there was an AVG anti-virus running on the background and the AVG icon was missing on the Task Bar. I decided to uninstall it but could not do it because of the same “permission” problem when opening “Add/ Remove Program” in Control Panel. I managed to get it uninstalled finally using our service program. After 2 hours hard work, the problem was fixed even without rebooting!