There was a recent post on this subject within the forums. I`m uncertain of the date or source but made a note for my own records; I have copied it below and have used it myself - it works.
"To speed up shutdown of the PC the value of this HKEY can be reduced. Often it is set higher than 2000.
1- Start/Run and type Regedit 2- Navigate to HKEY_LOCAL_MACHINE/SYSTEM/CURRENT CONTROL SET/CONTROL 3- Click on “Control” [highlight] 4- Select “Wait to kill service timeout” [in right hand pane] 5- Right click and select “Modify” 6- Set to a lower value than 2000 7- A setting of 200 will be sufficient."
Check the Application tab in Event Viewer to see if you're getting any 1517 or 1524 errors red crosses, that mention Windows saved user <user name> registry while an application or service was still using the registry during log off. The memory used by the user's registry has not been freed. The registry will be unloaded when it is no longer in use.
The user profile hive cleanup service may help in that case.