Users of WinXP and Win2000 frequently can connect to the Internet, but have problems getting File and Printer sharing to work properly. Sometimes it doesn't work at all, sometimes only some machines on a network can share, and sometimes machines can share, but only in one direction.
I can, and did, feel your pain, since I battled this problem myself and ended up reinstalling XP to get rid of it (yes, there's one tip right there). But this article will share tips for less drastic steps you can take to get your My Network Places happy happy again.
TIP: This article assumes that your network connection is basically working. If you need help making that so, try Troubleshooting Home Networking in Windows XP (Microsoft Knowledge Base Article - 308007)
This article will focus on WinXP, but some of the solutions - with modification - should work for Win2000, too.