Recently I had a problem upgrading a couple of SQL 2000 instances to SQL 2005 running exactly the same process as I had done many times before. Early in the update, I kept getting the error message:
UpgradeAdvisor: returned -1.
Alternatively sometimes it shows:
SQL BPA command line has stopped working
Turns out this is caused by the inbuilt upgrade advisor that runs within the upgrade setup, and fails if .Net 3.5 SP1 has been applied to the server.
The fix is simply to create a new directory and copy a single file (BPAClient.dll) within the setup bootstrap folder structure.
Either just search for the file name BPAClient.dll, or navigate to the BPA folder (probably C:\Program Files\Microsoft SQL Server\90\Setup Bootstrap\BPA) - the BPAClient.dll file should be in the "bin" directory in here.
Within the BPA directory, create a new directory called "BPAClient" - and into this directory copy the BPAClient.dll - now run the SQL 2005 setup to upgrade your instance and it should work.
UPDATE: Alternatively, just copy the BPAClient.dll file from the ...\BPA\bin folder into the ...\BPA directory - this also fixes the problem