Remote users are denied access when trying to access a share from Windows XP Professional

"Try again with all firewalls off.
Only if the problem still exists, here is some more:

Here's some useful documentation (although you probably know all this, it's always a good idea to go over the basics again while searching for a missing step). The first by Microsoft is pretty detailed, while the second is quite short and only intended for you as a flight-checklist:

How to configure file sharing in Windows XP
Share Files in Windows XP

If everything still fails, you might alternatively opt for sharing via the Shared Documents folder, which doesn't require too many permissions."

"Are both the machines in the same workgroup? Try going to a command prompt and typing

net use <driveletter>: \\<remotemachine>\<sharename> /user:<remotemachine>\<username> <password>

using any available drive letter and see if that works, if not it should give you an error message with a number which will give us some clues as to why not"