Many thanks to Stephen & Paul. Following your suggestion, I tried unmapping J: _before_ disconnecting from the LAN, and found that LyX 1.4.1 launched just fine. In my case, J: is a rarely-used network drive, so I unchecked "reconnect at logon" and will take pains to disconnect it after use in the future. The tip re/ modifying lyx.bat is also much appreciated.
