You should not use XSlimmer (or any other similar application, such as Monolingual) with Office 2008. Those applications take out files that are necessary to running Office 2008.
If you want to reduce the size of your Office installation, you should uninstall it, and reinstall it with a custom installation. During the custom install, you can choose to not install the things you don't use. For example, I never install the various language proofing tools. You can also choose not to install all of the applications that are included with Office 2008.
Regards,
Nadyne.