There is a problem with earlier versions of Apple System Profiler and the PowerBook 3400 that causes the application to hang when it is launched if the CD-ROM drive is inactive. This is fixed in version 1.2.1 of Apple System Profiler which comes with Mac OS 8.0 and is posted at Apple Software Updates sites.
Also, you can work around the problem by removing the CD-ROM drive or inserting a CD into the drive before launching Apple System Profiler.
These articles can help you locate the software update mentioned here:
"
Where To Find Apple Software Updates" - Lists online services for free Apple software updates.
"
Obtaining Apple Product Support in the USA" - Lists 800 numbers and online services for software updates, Apple support information, and a subset of the Apple Tech Info Library.