I have two laptops at work, both connected to the same network. One is quite happily connecting to Stack Overflow, the other gets 403 (unauthorised? Can't be bothered to check).
The main differences between the two laptops are:
Working laptop: Vista, proxy IP 220.127.116.11 Non-working: XP, proxy IP 18.104.22.168
The non-working laptop also routinely runs fiddler, but the fault happens with or without fiddler, on all the browsers installed, and continues to fail even though I've cleared cookies / cache. The only thing I can assume is that the proxy's IP has been banned. This affects most of the Stack Overflow family, including meta, and superuser.com.
What did I do?