If by "Default User" you mean "the user profile that will be used as the template when a user who otherwise doesn't have a cached or roaming profile logs-on" then you're talking about mounting the "NTUSER.DAT" hive from the "C:\Users\Default" directory and searching it.
There is a "HKEY_USERS.Default" in the registry, but that's NOT the registry used as a template for newly-created profiles. (It's actually the HKEY_CURRENT_USER hive for the SYSTEM context and really, really unfortunately named...)
The "REG.EXE" command can be used (with the "LOAD" argument) to load the hive in that file into the registry, then you can use your run-of-the-mill VBScript functions to access it. Afer you're done, you'll need to do an "UNLOAD" with the "REG" command to unload the hive.