Well, if you are thinking of connecting in from outside the router network
each PC will need it's own listening port and you will need to direct
connections to that port in the router setup / firewall.
Here is how to change the port for a start
http://support.microsoft.com/default.aspx?scid=kb;en-us;306759
You can only access XP Pro and W2003 machines in this way, the setting to
allow this is in the properties pages, "Remote" tab.
To access from outside you can use anything from 95 up, we have done it.
On one of the target machines search for mstsc*.* and copy the two files
mstsc.exe and mstscax.dll to the system32 directory on the machine you want
to use as client, these will already be there on XPpro and W2003 of
course... and then simply create a shortcut from the .exe file using "Send
to desktop".
Insert the ip address and port number xxx.xxx.xxx.xxx

ort (with the colon
of course) and away you go, hopefully.
None of the target machines should have users with weak passwords, if there
are any then don't permit remote or remove the user or change the password.
Note that using a lower resolution screen on the client may "Re-arrange" a
desktop and depending on the licensing method chosen a remote connection may
"Kick" a local user and vice versa.
Charlie