1) File and Print Sharing enabled on the Exceptions tab of Windows Firewall
2) Zone Alarm turned off or the appropriate exception for file and print sharing enabled.
3) Workgroup names the same.
4) enable Simple File Sharing (http://www.petri.co.il/enable_simple_file_sharing_in_windows_xp.htm)
5) something needs to be shared.

Things to try:

1) From a command prompt try pinging the other computer
2) Same command prompt try "net view" without the quotes.