Searching the net with google I found the string : "PGZvbnQg" appearing in a lot of web pages, mostly in, what I belive to be, encrypted emails that all begin with something like :
Content-Type: text/html; charset="iso-8859-1"
(just search with google and take a look)
so I was thinking that maybe is not just a simple encryption done in VBScript. Then again, it may all be a coincidence and I may be wrong.
Hope this helps.