Why not just use hexadecimal (0123456789ABCDEF)? That way you can just use a number (you could start at 1 or wherever satisfies your needs) and then just use the "hex()" function to convert the number to hex. That'll work just fine in most cases (and give you numbers and letters most of the time).
I'm sorry, but I don't think ASP came complete with a base-10 to base-36 number converter...
I'm pretty sure there is a way to do what your talking about though, I just have no idea on how to accomplish it. Somebody here should know how to do it.