The second of the two methods here does what you're wanting. I've just used it (after finding no success in this thread) and it's worked for me.
http://yorch.org/2011/10/two-ways-to-check-if-a-registry-key-exists-using-vbscript/
The code:
Const HKCR = &H80000000 'HKEY_CLASSES_ROOT
Const HKCU = &H80000001 'HKEY_CURRENT_USER
Const HKLM = &H80000002 'HKEY_LOCAL_MACHINE
Const HKUS = &H80000003 'HKEY_USERS
Const HKCC = &H80000005 'HKEY_CURRENT_CONFIG
Function KeyExists(Key, KeyPath)
Dim oReg: Set oReg = GetObject("winmgmts:!root/default:StdRegProv")
If oReg.EnumKey(Key, KeyPath, arrSubKeys) = 0 Then
KeyExists = True
Else
KeyExists = False
End If
End Function