The way I understand an "unknown error 31" indicates that the initiating PC is supplying a bad password or no passwords. So maybe the PC that can't access the XP machine is giving crap credentials.
I don't know for sure, this is just what I've gleaned from google and the Microsoft Knowledge Base, so there you go.
http://www.wown.info/j_helmig/wxpwin9x.htm#error31
I would check the network properties for the machines that can access the XP machine and compare it to the machine that can't. Do you have to logon to the other 98 machines?
Here's a page I found about it. It seems to sum up what little information I could find on the problem either on the page directly or with links.
I used to run a 98 machine hooked up to my XP machine upstairs. I didn't have any major problems. And, for some odd reason, the one time I had difficulty accessing the XP machine it straightened it out by itself. Well, I ramble....
good luck