Microsoft have been looking into this issue, and some ideas have been floated through newsgroups on how to solve this.
I've put together a couple of articles about this, which might help you get around the problem should it be happening to you. Unfortunately they're not all that straightforward...
- FAQ Article 547 - this is a collection of a number of suggested solutions
- FAQ Article 549 - a specific fix for CD installations only
If you're experiencing the problem, hopefully something in one of those articles will help you.