I have used double " instead of single " and its working fine for me.
Set oShell = CreateObject ("Wscript.Shell")
Dim strArgs
strArgs = "cmd /c REG ADD ""HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\RUN"" /v ""MyCustomWorkgroupUsername"" /t REG_DWORD /d 0"
oShell.Run strArgs, 0, false