Here's the VBScript code that you'll need.
'begin script
Dim objShell
On error resume next
''''''''''''''
' Registry key
''''''''''''''
strRegLoc = "HKEY_LOCAL_MACHINE\SOFTWARE\_
Microsoft\Exchange\Client\Options"
strKey = "<keyname>"
strValue = "<value>"
''''''''''''''''''''''''
'write new key and value
''''''''''''''''''''''''
Set objShell = CreateObject("WScript.Shell")
objShell.RegWrite strRegLoc & strKey, 1, "REG_SZ"
objShell.RegWrite strRegLoc & strKey,_
strValueLocal, "REG_SZ"
'end script
Obviously, you'll need to enter your key name and value into the given script.
Then, create a new GPO, remove all administrative templates for faster policy processing. Go to the scripts section under the desired node (user or machine). Add this new logon or startup script. You'll need to place the .vbs file into its respective GPO directory.
Hope This Helps,
Good Luck!