This happened to me too and apparently, is a problem with the student version. Arnoud helped me figure out a work around for this. In your
$UserBaseDirectory/Licensing, you should find a
mathpass file that looks something like this:
machineName1 ID ActivationKey1 Expiration1 UserName
machineName2 ID ActivationKey2 Expiration2 UserName
The first line is (most likely) your activation for the trial, and Mathematica 8 for students apparently tries to use this activation key and converts it to a trial version.
In my case (OS X), Mathematica 8 kept crashing, and Arnoud suspects it's because it tries to bring up the trial window at startup, which does not exist. You seem to not have that issue, but the workaround (for now) is to comment out the activation line (with a
%) for Mathematica 9 trial version when you're not using it and then launch Mathematica 8.
Oh, and the problem seems to be primarily with the front-end, as I can launch a Mathematica 8 kernel from the command line simultaneously with Mathematica 9. So if you only need it for computations, that's another option.