Install Office 2008 in addition to Office X. It will create a separate folder and file hierarchy.
During the Office 2008 install process select custom install. Then remove the install selection from everything except Powerpoint. I don't know whether that will cause conflicts or not but you can not install Powerpoint 2008 and add it to Office X. You must install Office 2008.
Is there any reason you don't want to upgrade the complete Office Suite?
Regards.