I have exactly this problem too!

Some facts about this problem:
- IE and other browsers stop working for no obvious reason
- Not even local web servers can be reached (like 192.168.0.1)
- This can happen at any time, not correlated to any special program being used
- *Sometimes* (very seldom) it starts working again, after 5-30 minutes
- Microsoft Mail stops working at the same time.
- Windows Update stops working (it still finds new updates and say "There are new updates" and so on, but when trying to install then it bombs out with a 8-digit hexadecimal error code).
- The help text for the error code is wrong. It is not a dns problem or a connection problem.
- MSN/Windows Live Messenger stops working *sometimes* but not always
- I can still ping hosts on the internet, and DNS works just fine
- This happens on Vista (I have Vista Business)

What i have tried:
- Logging out and logging in again: does not solve the problem
- Disconnecting and reconnecting network: does not solve the problem
- Restarting router etc: does not solve the problem
- Disabling the windows firewall: does not solve the problem
- Disabling and enabling network interface: does not solve the problem
- Suspend/sleep mode and wake up again: does not solve the problem
- Stopping and restarting all "network"-ish services: does not solve the problem

What works?
- Reboot. Nothing else.

What this is NOT:
- This is not a router problem/external network problem
- This is not a DNS problem (not external, not internal)
- This is not a network adapter driver problem
- This is not a hardware problem
- This is not a bug in IE
- This is not a bug in the DNS cache service

So, what is this?
- This is a bug in Vista. A very annoying one!

If someone at M$ reads this, then PLEASE do something about this.
I am thinking about deleting Vista and installing XP or something. This is just to sucky.