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 applications, and having no clue what the service does, I tried disabling it. To my honest surprise, it resolved the issue. The service was indeed effectively blocking startup of other services, including those required by networking.

As an alternative, you could also uninstall the entire .NET 4, although this method is much more quicker, and Windows Update does not bug you trying to install it again. Of course, downside is you have to keep in mind that you've disabled the service, although I suspect when Microsoft releases an update to fix the issue, it will automagically be reactivated.