How do I troubleshoot HTTP 403.1 Forbidden?

All of the sudden I started getting 'HTTP 403.1 Forbidden'
whenever I access any *.ASP or *.ASPX pages from the web
server.

If I run 'Check Server Extensions' I get 'The folder
Web/_vti_bin' is not executable. Making it executable',
but if I run the same check again, I get the same error
message.

It sounds like FP server extension problem, but I don't
know how to troubleshoot it further...

Thanks,

-Stan