For some reason, when running the SQL Server 2005 Upgrade Advisor (either the stand-alone version or the version that runs within the installation), I was getting the following error:
SQL BPA command line has encountered a problem and needs to close.
The problem, it turns out, is that BPACMD.EXE was not able to load BPAClient.dll for some reason. The work around was to copy the BPAClient.dll to a location that allowed BPACMD.EXE to load it. The file resides in a “bin” directory below a “BPA” directory. I simply created a “BPA\BPAClient” directory and copied BPAClient.dll into this directory. (The reason this works is that the .NET runtime will search this directory when trying to load BPAClient.dll.)
Note that you must copy this file after the SQL Server support files have been installed (usually to the Program Files\Microsoft SQL Server\90\Setup Bootstrap\BPA directory).
A forum discussion of this topic is here.
I am not sure the reason this intervention is required, but I suspect it has something to do with the installation of the .NET 3.5 SP1 runtime.
Second Stanza 
Thank you ! This advice helps me so much :-)
Comment by Steve Branney — August 8, 2010 @ 8:23 am
Thanks! You’re a lifesaver!
Comment by Ryan — January 14, 2011 @ 3:54 pm
Thank you very much. This saved me hours of wasted time.
Comment by Wes — April 1, 2011 @ 11:14 pm
SUPER DAAAANNNKKE, auf so etwas kann man doch nicht kommen.
Habe mich dann auch durch die logs gehangelt und dann noch diesen Beitrag gefunden.
Danke schön. So einfach und schnell kann es gehen.
Comment by Hubert — April 30, 2011 @ 6:23 pm
Thank you! you saved us a lot of time!
Comment by Ram — June 3, 2011 @ 12:24 am
You are THE MAN!!!!!
SP4 for SQL2005 was failing, it drove me nuts.
I tried a ton of solutions – until yours solved it.
Thanks!
Comment by Non Anon — June 20, 2011 @ 8:02 pm