Since the Parallels KB's don't list this, I'd figure I'll post it here. You may run into the following when starting an existing Virtuozzo container on Windows.
C:\>vzctl start 100 --verbose
Starting container ...
INFO: Notify container 1: stopped starting
INFO: start_vps_locked: container 1
INFO: VpsVolumeMount() DISK_TYPE = 3 : 2
INFO: VpsVolumeMount() DISK_TYPE = 3 : 2
INFO: Completed successfully VzhiveMount: "D:\vz\Private\100\root\vzquota" "D:\vz\Templates" "w2k3″
INFO: Applying patch to D:\vz\Private\100\root\WINDOWS\system32\win32k.sys...
INFO: Patching security database...
INFO: Done!
INFO: (RetCode = 0, CT IP=x.x.x.x, Gateway IP=x.x.x.x, IfId=0×4)
ERROR: RegOpenKeyEx returned 2
ERROR: Unable to copy key HKLM\SYSTEM\CurrentControlSet\Services\SRTSP to HKLM\vz100\MACHINE\SYSTEM\CurrentControlSet\Services\SRTSP
ERROR: Virtuozzo API function call 'VzkrnlStartVps' failed dwErr=0x0000013D
INFO: stop_vps_locked: container 1
ERROR: Container 100 is not started
ERROR: Exec '@VzOnShutdown' failed in container 100
WARNING: Virtuozzo API function call 'VzkrnlStopVps' failed CTID = 1 Error = 0×00000490
INFO: RetCode = 0, CT IP=x.x.x.x, Gateway IP=x.x.x.x, IfId=0×4)
INFO: Notify container 1: stopped none
The exact error is caused by the "Unable to copy key" line, hidden in there. Open your regedit and browse to HKEY_LOCAL_MACHINE/SOFTWARE/SWsoft/Virtuozzo/AV and see if any entries exist in that particular folder.
If there are, and you've recently uninstalled an Anti-Virus application, you can try to rename that folder AV to something else, and restart your Virtuozzo Container.