SQL Server 2005 Express: upgrading to SP3 in mixed-mode installations

I’m having trouble upgrading SQL Server 2005 Express SP1 to SP3.

The SP1 install uses mixed mode authentication (so there is an sa password).

This is the message I get:

TITLE: Microsoft SQL Server Setup
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None of the selected features can be installed or upgraded. Setup cannot proceed since no effective change is being made to the machine. To continue, click Back and then select features to install. To exit SQL Server Setup, click Cancel.

For help, click: http://go.microsoft.com/fwlink?LinkID=20476&ProdName=Microsoft+SQL+Server&ProdVer=9.00.4035.00&EvtSrc=setup.rll&EvtID=SQLSetup90&EvtType=28108
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BUTTONS: OK
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The link then tells me To continue you must provide a strong sa password.

I tried some searching, and found something about BPAClient.dll, but this batch-file does not fix it:

mkdir "%ProgramFiles%\Microsoft SQL Server\90\Setup Bootstrap\BPA\BPAClient"
copy "%ProgramFiles%\Microsoft SQL Server\90\Setup Bootstrap\BPA\bin\BPAClient.dll" "%ProgramFiles%\Microsoft SQL Server\90\Setup Bootstrap\BPA\BPAClient\"

I also tried this, but that didn’t help either (probably because the services had been started at least once before trying the update).

So I think the clue is the strong in the link above.
Am I on the right track?
Where do I find more information on the strongness of an sa password?
Duh – that was in the above link. Just didn’t read well enough.
So I changed the sa password to something strong (Upper and lower case letters, special signs, at least 6 characters).
Rebooted, restarted SP3 install: still fails.