Had been having a rather irritating issue with Plesk for Windows that prevented domains from being deleted. Before wiping the system clean after fruitless attempts at searching out a solution on the Plesk support forums, I finally found the answer (thank you Google).

If you are using Plesk for Windows and getting this message :

Error: Unable to remove domains: Problems occured while removing domains: Unable to remove hosting: Unable to delete system user: SysUser->systemRemove() failed: usermng failed: Unable to open process token: (5) Access is denied.

The solution is to disable process-level tamper protection in SAV by: Start > All Programs > Symantec Client Security > Symantec Antivirus > expand Configure > Tamper Protection > Uncheck Protection: Processes.

Reference :
Plesk 8.3 incompatibility with Symantec AntiVirus 10.2 (and a rant about Parallels!)