>>>When I installed IIS on the server, I configured it to support multiple sites. Some of those sites have ASP pages. The main site is working ok. However, all other sites can't display ASP pages. It returns error 500 as if the page wasn't there. I've been looking for 2 hours to try to find the difference in those Web site configurations and couldn't find it. What may cause this?
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>>Do they have the FP extensions installed?
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>I don't remember having done such configuration for my main site. Where do I go to install that?

I'm not sitting in front of my server, but right click the site-->properties-->home directory (I think)-->then click Front Page Web. This is from memory, but it should be close. There is also a setting in the Default Web Site that says apply fp extensions to all new sites - or something similar.

HTH