Sorry that didn't help. Here's a few other thoughts and some questions.
If you try to map a drive on one of the other comptuers with the net command
can you then access it in Windows Explorer?
(If you're not familiar with this, open a command prompt and run >net use
/?
An example of use is >net use Q: \\pcname\C )
You indicated you were able to "re-establish" the connections when they
failed earlier. Did they just come back after rebooting, or did you do
something in partiular?
This problem was intermittent before, but now has lasted for 2 days?
Have you double checked that the XP firewall is not enabled? (I realize
you didnt change this, but would still double check it.) Also, if your
router has a built in firewall, you might want to see if anything has
changed. Also, it may have a log file that would give some indication
Can the other machines see/access the XP machines folders and each other?
Are you running XP Pro or Home?