if using winxP you must disable your winXP firewall. it is in my computer>> control panel>> network connections>> network connections (no thats not a repeat, winXP is user over-friendly).
right click on your internet connection and select properties. look around and your will see a box that says "protect my computer and network". thats winXP firewall, uncheck the box and accept changes.
while still in the network connections window right click your connection again and select "disable", wait a few seconds and do the same thing and select "enable".
if you have any other firewall besides winXP that would block file sharing too unless you configure it or disable.