The problem is perfectly described in the URL. Win XP doesn't send host
announcements like it should , even if you edit "announce" and "anndelta" in
the registry. 
It would be nice if you can tell samba to keep the entries in browse.dat for at
least $time seconds.
Btw, this is an old problem:
There are some more sites. Win XP seems to disappear in a pure Microsoft
environment as well sometimes.
I solved my problem so far. I found out, that WinXP at least reacts to "Host
Announcement Requests". Samba generates them while restarting. Samba does it too
after winning the LMB contest IF there are less than 10 computers in the
workgroup. So I patched my samba server and added "smbcontrol nmbd
force-election" to my crontab. This gets done every 10 minutes. Now, WinXP does
not disappear anymore. This is really ugly...
 WinXP ignores quite some registry workarounds...
 I have more than 10 computers in my workgroup.
WinXP client bug.