My apologies for not trying the msconfig test, because that solved it
Turns out my Creative X-Fi soundcard has a service called "Creative Audio Service" that was the cause.
Just stopping the service manually while Windows is running is enough.
Sound works fine when the service is stopped, so not a real problem.
Maybe the support guys can use this in future troubleshooting or fixing it, since the problem apparently doesn't exist in ToMI 1-4