I think that's probably due to three things. Reduced disk space, much larger registry, and disk/swap file fragmentation, only one of which you really have any control over.
What I would do is uninstall Office, delete all temp, browser cache, and unnecessary files, empty the recycle bin, defragment the hard drive, set the swap file to a fixed size (~500MB), install Office, defragment again. That may make a moderate difference.