I'm sure only HP do that dirty trick.
There was a bug with the original 2007 uninstaller.. in that it didn't remove all files. If you were using Office 2003 VLK for example at work, you had to uninstall the 2007 trial, and install 2003 (obviously).. but it caused crap with the files it left behind!
One major problem was Outlook 2003 failed miserably.. wouldn't connect to any Exchange server without deleting some files manually..
Annoying as hell. Took ages to find the problem and get round it..
AFAIK most manufacturers are now doing it, i'm sure i read ibm were too.
Its no dirty trick LOL, its the only way to provide OEM legally these days unless you are dell and sell the license with the machine as its built to order.
What confuses me is that machines that come with office 2007 trial on also have SQL 2005 components installed and i'm not sure what for, HP can't even tell me, must be something to do with one of the components of office, did a search and there was something about data mining in excel which is random.