Tip 5: If all else fails...
If you're still reading, then it's time for brute force. Here a few other things to try.
First, try a System Restore to a point before you started having file and printer sharing problems. If you've never had occasion to use System Restore before, it's located in the System Control Panel, System Restore tab.
Next, you can try resetting the TCP/IP stack. In Win98 you could just uninstall and reinstall the TCP/IP protocol from the Network Properties, but, once again, XP has its own way of doing things. See the How to Reset Internet Protocol (TCP/IP) in Windows XP (Microsoft Knowledge Base Article - 299357) for the how-to. [Thnx to Ethan Frome for this tip!]
Finally, if all else fails, then, what the heck, do an XP reinstall. In my case, I didn't even do a clean install (including a disk format), and it worked like a champ!