When running Plesk 8.3 (Windows) with Symantec AntiVirus (SAV) 10.2 (on Windows 2003 R2 SP2), you will receive the following error message whenever you delete a domain: "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.
Hopefully someone else won't need to waste as much time as I did fully reinstalling the server using an iterative process to find this solution.
On a side note, Parallels (company that makes Plesk) was entirely UNHELPFUL and UNCOOPERATIVE! After waiting 3 days for a response, I was told that my Windows was corrupt and to wipe it and start over. Only after my own diagnosis was I able to identify this issue. I don't know who to blame here because SAV shouldn't have been so aggressive toward Plesk's code, but Plesk should have ensured SAV didn't identify their code as an attack. But I do know that Parallels could have treated me MUCH better and been much more helpful.
I tried to identify a competitive product to buy instead, after this experience, but it appears Parallels has bought the entire market! Amazing!
One final note: If you're thinking of using Ensim Pro (as we were), don't bother. It is truly an old and inferior product as compared to Plesk. There have been no real updates to it in 5 years! While they market it as a single-server version, it is not even close. My sales person at Parallels, Terry Henretty, even told me that they never sell Ensim Pro, that they just keep it around for the current users and that everyone ends up buying Plesk. But for 134% MORE money, I'd call it bait-and-switch. When I contacted three Parallels executives, Jack Zubarev, Serguei Beloussov, and Kurt Daniel about this tactic, I received no reply whatsoever. Be warned!