With the push of .NET Framework v4 through Windows Update, I've noticed a problem with Windows XP clients. They all suddenly started to boot up slowly, i.e. it takes an abnormal long time before the network stack is started.

Logical deduction and analysis of the problem lead me to check which services were taking a long time to start, as this could be an indication of cause. After rebooting, then immediately digging into Services (services.msc) shows a service trying to start, called Microsoft .NET Framework NGEN v4.0.30319_X86.
As the users of the affected computers are not using any .NET 4   ...