As Tycho's questions made allude to you, we definitely need to know more in order to be more helpful. Sometimes, when I run into this, it can be one of the following:
- The URL provider and/or it's setting(s) in the web.config file (compare yours to a clean installation of the same version of DNN)
- Bad PortalAlias settings in the PortalAlias table in the database (also check the default alias in the PortalSettings table).
- A rogue module and/or HttpModule that's controlling URL's.
One thing I sometimes do to help troubleshoot this is to use a tool like Fiddler to instead the requests and responses after turning on debugging (in DNN itself and in the web.config). It doesn't always help. That really depends on the underlying issue that's causing the redirects.