Xp PC showing up intermittently on Vista network

>I have a small network setup at home that has one XP Home pc, one Xp pro
>and
> one Vista home premium PC. The two Xp pc's have no problem sharing folders
> or
> showing up in network places. However the XP home shows up on the Vista
> map
> intermittently even while I have mapped drives on the Xp home that Vista
> is
> able to access. I have the lltd protocol installed. The Xp pro seems to
> always be available to the Vista machine. So my problem is why does this
> pc
> show up intermittently? Sometimes the mapped drives are unavailable to but
> usually just can't see it on the network map.
>