Backup puzzle

I am attempting to set up a simple backup scheme using two workstations
at a local small business. Both are running XP (tho one is home and one
is Pro, to the surprise of the owners) and they're on a network. My
goal is to back up the "My Documents" directory of each onto the other
via the network.

To accomplish this task, I created a directory named "backup" on each
system then mapped a drive on each to the other's backup directory. I
then configured the Microsoft backup utility to run a scheduled backup
to this mapped drive.

One of the systems works fine.

On the other, the backup starts normally and hangs immediately in the
process of scanning to determine which files to back up. There's no
error message. If you interrupt the backup, it ends without complaint,
but does not create a file.

Today I decided that I would solve the problem by changing from the
Microsoft-supplied backup utility to Retrospect Express. After
installing and configuring it, I experienced exactly the same symptom.
The screen that normally shows the files being scanned came up, and then
the backup simply hung.

At this point I conclude that the problem does not lie in the backup
software, but instead in some locking mechanism that is causing the
process to hang waiting for something to complete. It's not a problem
writing to the other workstation via the network -- I can copy the files
across the network just fine using File Explorer.

The failing system is running MS SQL. I am not trying to back up any
files that are associated with MS SQL, but this is a difference between
the two workstations (the one that's working is not running MS SQL).
Other than that, I can't report any differences between the two systems.
I'm sure there are many differences, as there always are between two
"identical" workstations, but I can't point to anything that would
explain why the backup works on one system but not the other.

I would appreciate suggestions on what the problem might be, but even
better would be ideas on how to proceed with troubleshooting. It's
tough when the program just hangs with no error indicated.

Thanks.