Mpg wont import into Sony Vegas 8.0a properly, if at all.

Mpg files wont import into my registered and activated Sony Vegas 8.0a build 179. Some import without audio but most won't import at all, giving the error: "Warning, an error occurred while opening one or more files. The file is an unsupported format." Then it has the file location.

I should imagine I'm missing a plugin, if that's the case, could somebody tell me which?

Thanks in advance.