pings both ways but WExplorer only shows shares on one machine

Have an XP desktop with shares that can't be seen on Vista laptop.

I can PING netbios name of other machine on each PC and reply comes back immediately.

WExplorer of XP machine shows Vista shares

WExplorer of Vista machine shows XP name but when you try to access, you get user/password dialog which does nothing but come back again when filled out properly

Same users and passwords on both machines

Same workgroup name on both machines....

If I enable the Guest account on the XP machine all works OK... I don't want this account enabled because of bad virus and rootkit experiences in the past... shouldn't have to leave this door open if networking is set up properly.

Any thoughts?