Sometimes ago I managed to install a demo version of nod32 server management console and a few clients then during the initial configuration I add a simple password to make it more secure and as you may guess by the title the password did not work later so the anti virus wouldn't uninstall!.
after a few search in forums I came up to this solution :
1- open registry (Start > Run > Regedit)
2- follow the keys to this value
3- right click and rename the PackageID thing to something else like packageID1 (you may have to restart in safe mode to be able to change the name)
4-restart and it's done, now you can uninstall it