I have a very odd problem that I can't resolve. I am connected to the internet, but my browsing doesn't work. I don't mean a web browser - I mean browsing. Firefox, Chrome, Curl all fail to successfully connect to an HTTP address.

However existing connections, e.g. to mail in Outlook (Exchange Server and also IMAP server) continue to work.

Also, the internet is on, I can confirm both from my machine (other ports / connections) as well as from any other computer connected to the same network.

Additionally, it appears to be HTTP, not simple a port issue as HTTP over port 8443 (Tortoise SVN if you must know - running over HTTP not over SVN) also fails.

I am using Windows Vista SP2 (build 6002). It seems to "creep up" in that after running the computer for a few hours it will fail. (No found way to systematically reproduce the problem.) Additionally, it seems to be more prone on days where the internet connection is flaky already (not sure why the internet is flaky, just is, lot's of failed browsing requests and have to retry/reload often).

What I have tried (when the problem arises) - none have yielded any resolution:

• Resetting the network connection (dis-connect, re-connect)
• Disable/re-enable the network adapter
• Double-checked the ip settings
• Double-checked the HOSTS file. Note: DNS continues to work (both new and cached responses to DNS queries). (Thanks for the suggestion Daniel and antenore.)
• Checked the routing tables (ip4 only as ipv6 is beyond my understanding)
• resetting all involved hardware (routers and modems)
• Close and reopen browsers
• Looked for malware interference:
  • Run HijackThis
  • Looked for suspicious processes using SysInternals procexp.
  • Looked for explorer hijacks, lsa provider interference, winsock provider interference using SysInternals Autoruns.
  • Run a complete anti-virus scan.
• Reviewed the output of a netstat -onab to see if there were stuck ports open or unusual processes running somewhere

The only thing that works is to do a full reboot. That works 100% of the time to restore browsing.

What else can I try to nail down the problem?