IIS7 .NET (2.0, 4.0) completely hosed with a Bad Request (Error Code 400) error

All of a sudden all of my websites on my server return 400 Bad Request Error. I don't have a clue what happened. App Pools are running in Classic pipeline mode (4.0,开发者_JS百科 2.0), doesn't matter.

Every URL that I type comes back as 400 Bad Request. Real URLs, even fake URLs that don't exist (which should come back as 404) all are 400.

http://mywebsite.com/AFile.aspx
http://mywebsite.com/AFolder/AnotherFile.aspx
http://mywebsite.com/Bfolder/YetAnotherSillyPage.aspx
http://mywebsite.com/A_stupid_URL_that_does_not_even_exist_fjfjffjfj.aspx

Everything 400 Bad Request. Totally screwed up my ASP.NET. Where should I begin to look? Machine.config? Web.config?

UPDATE: After trying a million different settings, I finally set the App Pool to Integrated and set the Identity to LocalSystem and all of a sudden it works.

Bad Request usually is HTTP.sys stopping the request due to something really bad (like invalid URLs, or something like that). You probably should look at HTTP.sys logs (Not IIS) at:
C:\Windows\System32\LogFiles\HTTPERR

Also, maybe something got broken in the http.sys configuration so try running:
netsh http show servicestate

And see if for your web site it has the correct bindings, for example it could be that the bindings are only listening on only specific IP Addresses and yet its coming from another one, or similar problem but with Host Name, etc.

Finally you might want to run:
C:\Windows\System32\inetsrv\appcmd list sites

And see if the bindings and status makes sense.

Have you tried some mixture of re-installing (or uninstall/install) asp.net using the aspnet_regiis.exe utility? That's fixed strange IIS/ASP.NET server issues for me in the past.

Have you looked in the event log for any error messages or further clues?