How Can This Be Done? IIS Multiple Vir. Dir. Or Real Folders

The scenerio is client want to enable unlimited URL for his individual customers.
ie,[url]
Platform: W2K3 IIS6

I only know two ways doing it

1. Create a real folder /username1, /username2, but this will be real messy, and I remember there is a limitation for up to 36,000 sub-folders within a root folder under Windows (correct me if I am wrong)

2. Create virtual directories under IIS Manager using ASP/ASP.NET script, this is easiest, but having two problem.

a. If I have say 10,000 vir. dir., and then I try to expand that root folder under IIS manager, IIS manager will hang for sure.

b. Having such huge vir. dir will inevitably having a huge IIS Metabase, this means a great chance of corrupting it, so it's very dangrous.

I really hope someone can give me some hints how to do this in a scalable way? I know many Web2.0 site do this even using IIS6 ie, [url]