During the process of migrating a site to IIS 7 we came across an issue with Classic ASP file uploads throwing 500 errors when larger than 200kb. There is an EASY fix for this.
In IIS 7, click your site and expand it then click the ASP icon.
Expand the Limits Properties icon, and change the value in the “Maximum Requesting Entity Body Limit” to a value larger than 200000 (which is about 200kb). 2000000 would be roughly 2mb, 20000000 would be 20mb.