Further to my previous post, I've come across more issues with HTTP compression and IIS 6.0. IIS allows the compression of dynamic files (e.g. files generated on the fly by ASP.NET handlers), but Internet Explorer doesn't appear to like it when the dynamic file is a zip itself. Unfortunately, compression is controlled at the "Service" level (right-click on Web Sites, rather than right clicking on an individual web site) in IIS manager snap-in, and using shared infrastructure can mean that your web site breaks immediately for no apparent reason. Lucky for you though, the metabase can be tweaked to enable/disable compression at the "Site" level... you just have to be able to figure out the site number.
adsutil.vbs SET W3SVC/687245286/root/DoDynamicCompression False
adsutil.vbs SET W3SVC/687245286/root/DoStaticCompression True