PCs not showing up in ME's My Network Places is a widely-experienced 'bug'. The solution is to map a drive on the ME PC to the XP PC(s). For some reason, ME seems to have problems (on some installations) with browsing of My Network Places.
See the detailed discussion at click here
I suspect the problem with the laptop may be firewall-related. If it can see them, but is denied access, that is the most common cause.
If the suggestions above do not help, can you clarify whether you are using XP Home or Pro?