Many people including me have been tearing their hair out trying to resolve this problem with Vista.
The error occurs when entering sleep mode on Vista 64bit OS, upon resume the network card is powered and appears to be active, however no network or internet connection is available.
The various fixes I tried which did nothing included:
1. Running MS update for patches (none found)
2. Updating NIC drivers (NVidia nforce)
3. Updating MOBO BIOS
4. Disabling IPv6 for all network adapters, including disabling IPv6 in the registry
5. Every conceivable combination of settings for power options in the OS and on the NIC, both hardware (e.g. WOL) and software (allow windows to control...)
6. Reviewed output of ipconfig /all both before and after sleep (no changes)
7. Locking IPs, subnet, default gateways on the NIC
8. Praying, voodoo, runes, etc.
In the end, the simplest of fixes sorted this out. I disabled and uninstalled the NVidia NIC via control panel>system>device manager, then rebooted.
Vista loaded up the original MS drivers for the card, and bizarrely it works perfectly now, instantly reconnecting after sleep.
Hope it works for you as well.