chipmafia,
Go into the device manager, open up the network hardware, right-click on the Ethernet adapter you're using and click Properties. Click on the "Configure" button, then on the "Advanced" tab - go through the Property list and document all of the settings for each parameter. Also, click on the "Power" tab and make sure ALL power management features are disabled.
Dumb question and hopefully it won't insult you - but have you tried swapping the Ethernet cable between the PC and router with a different cable? Have you tried connecting to a different port on the router?
Check this out:
http://www.neowin.net/forum/index.php?showtopic=496371
There seems to be an issue with the Vista drivers for your motherboard's onboard Ethernet controller - and the WOL (Wake On LAN) settings. Read this post and it kinda sounds similar to the issues you're having.
One quick solution would be to disable the motherboard's built-in Ethernet hardware - go purchase a PCI Ethernet adapter (I like Intel's) that has driver support for Vista. Install this and you could see this problem go away.
- John