Are you running it along with other installations of IE?
I am running it along side of IE8, Expression Web Super preview, and IETester.
All are working with no problems that I have noticed. One other thing though IEFilters do not work in the preview, but flash swf/flv's do, the IEFilters problem is known, and is to do with the fact that I have IE8 installed and this uses the vectors for the filters.
I also intend to update my advanced css2.1 / css3 test page to see exactly what does work over the next few days, if you are interested in testing the IE9 preview for yourself in this respect the link is - http://www.pziecina.com/design/turorial_demos/css_demo.php.
Update to using the IE preview.
If the page you wish to view does not appear in the IE9 preview, switch to another application then back to the preview.
This post is IMPORTANT as it may seriously affect your css code in IE9.
When testing in the IE9 preview, I found that some css code did not render, and upon investigation the cause was found to be the missing leading 0 before the decimal points.
From the release notes at - http://ie.microsoft.com/testdrive/info/ReleaseNotes/Default.html.
Which states -
Issue 541446: Floating-point properties won't parse values without a leading zero
Description: CSS floating point values which are less than 1, do not parse correctly if they do not include the leading 0. For example "0.5" will parse correctly but ".5" will not.
Workaround: Include the leading zero for floating point values which are less than 1.
This means that if you have included something like - font-size: .9em; in your css, it WILL be ignored in IE9, (Adobe, are you reading this, as much of your css includes such values).
For those who have often replied to posts advising people that the leading 0 is not required!!!
Thanks for that update. I typically use leading zeros on ems but it won't hurt to double check 'em.