Believe it or not, this night, I upgraded a SQL Server from 2000 to 2005.  During the upgrade, the following error was presented, luckily before any files were written:

Turns out, J-F Tourigny (thanks very much!) had the same problem when trying to upgrade from MSDE 2000 to SQL Express 2005.

“When trying to upgrade MSDE 2000 to SQL Express 2005, the installation would fail during the upgrade advisor with error -1.  In Application log, Event 5000 was generated for process “bpacmd.exe”.  A quick search on the Internet revealed that the process is looking for BPAClient.dll in the wrong directory. Creating folder %ProgramFiles%\Microsoft SQL Server\90\Setup Bootstrap\BPA\BPAClient and moving the DLL into this folder solved the issue”

The same solution worked for me when upgrading SQL from 2000 to 2005.