1. Try the network troubleshooter in the help. It will help you to check every setting, service and protocol involved.
2. Try system restore (Win XP) or scanreg /restore (Win 98), if you've can go back to a version that worked that way.
3. Try uninstalling and reinstalling and reconfiguring everything you did a few days ago. If it worked then, and you started from scratch, it should work now also.
4. If Internet works fine on both machines (my idea: you've got a router connected with 2 PC's, correct me if I'm very wrong) the hardware and drivers for the NIC should be OK. Which is a positive point.
I set my networking up a coupe days ago. It was working fine, i could get stuff from my other computer. Now all of sudden its not working. i havnt messed with any of the network settings or anything. Is there a reason why its doing this? the iternet works but i just get shar files. Thanks for the help!!