Server not authenticating social network APIs

I am trying to access Twitter and LinkedIn APIs, however I keep getting the error "401 Unauthorised".

I am pretty sure that the problem is due to something wrong in my IIS setup, as it was working when I previously had a shared server.

I've read that the problem could be due to time settings, and if my server's time is different to the API's server time, then it wil refuse my connection.
After trying different solutions, I've still had no luck.

Any ideas?