You see the other system in your "Network Neighborhood", select a resource and now try to map it, and then:
When you get such (or a similar "Mapping") error message, have a look to your CONFIG.SYS, whether you have in there a "LASTDRIVE" statement:
You CANNOT use a drive-letter a drive within the range defined by LASTDRIVE !
Solution: either delete the LASTDRIVE-statement in your CONFIG.SYS (it is in most cases anyway a left-over from an upgrade from a DOS to a Win95 system and usually not anymore required)
Use a drive-letter ABOVE the LASTDRIVE range.
If you like more details on this issue, please look at the Microsoft Knowledge base article Q130077:
Are you trying to map your own disk-drive as a network drive ?
see Sharing and Mapping for the solution