Well it could be due to a problem with the way that the system is configured for the logging policy for the Setupapi.app.log

You can fix this automatically with the following Microsoft Fixit

Microsoft Fixit 50162

read the full KB article (KB958909) that gives a lot more detail on this problem and how to fix it manually if needs be…

It takes a long time to log on to a Windows Vista-based computer that has antivirus software installed, and you notice that the size of the Setupapi.app.log file is very large

Learn more about how to Optimize Windows Vista for better performance

Rob