I had a similar problem in an old project that was using both
versions of Common Controls (5 and 6). I removed all the old
controls and I upgraded them to the new ones and the problems
went away. If you have this problem using only version 6, it
can be a bug in that version. Check the MSDN knowledge based
articles to see if there is anything. If you do not find anything
and your code is small enough, I am sure someone will be happy
to peek into it...

Marco