Help! Win 98 like server and Win Xp pro like client problem

Hi all,
I need a help, I have a peer-to-peer network with two windows 98 se, one
Windows ME and one Windows XP.
The XP is SP2 host and is a new PC. I've disabled on it the firewall and
I've abilitate Netbios on TCP/IP.
From the Xp machine I can browse the network workgroup, browse other win 98
and ME hosts, open files (Word, Excel etc.), save and modify this files,
create new files, so I can normally use the network resources.

One of the two Win 98 PC is a server for some vendors programs.
This is the problem:
All non XP clients (98 and ME) can access, open, modify and save the
vendors program files stored on Win 98 server with no problem, while the XP
machine can successful open, modify and save that files, but strangely for
just a program, when I open an existing vendor file and I modify it, then
when I try to save it, I can’t; it doesn’t save the modify! but I can create
a new file and save it with no problem!

So, there is a problem just for that program? it can be a network problem?
What for you could be the issue? Why only when I try to modify and save
again the file it doesn’t save it? The “only read� on the windows 98 PC
server files are unchecked…
What are happening?
Thanks to all that can suggest me some possible solution.

Beppe