Ok, so back to my original script. All fixed and working! 
Code:
strComputer = "."
Set objWMIService = GetObject("winmgmts:\\" & strComputer & "\root\cimv2")
Set colDisks = objWMIService. _
ExecQuery("Select * from Win32_MappedLogicalDisk")
For Each objDisk in colDisks
if objDisk.Name = "Z:" Then
If objDisk.FreeSpace = 0 Then
'Out of Space
MsgBox "You have no space left in your My Documents. Please delete any unneeded files.", vbCritical + vbOKOnly, "Disk Quota Error!"
Else If objDisk.FreeSpace < (objDisk.Size / 100) * 90 Then
'Less than 10% left!
MsgBox "You have very little space left in your My Documents. Please delete any unneeded files.", vbExclamation + vbOKOnly, "Disk Quota Warning!"
Else
'We're fine.
MsgBox "Free Space for your My Documents: " & Int(objDisk.FreeSpace / 1024 / 1024 ) & "Mb of " & Int(objDisk.Size / 1024 / 1024) & "Mb", vbInformation + vbOKOnly, "Disk Quota Information"
End If
End If
End If
Next If anyone else needs it.