Here's a way to do it. I tested this with office xp. I don't use the office assistant so I couldn't tell if all of the features were compatible with office xp, but the animation was fine and there were sound effects when he entered.
Go to this web site:
and download the following files in the directory listing:
Save or copy them to Program Files\Microsoft Office\Office10.
The only way I could get him to show up in the options to change office assistant was by renaming the files to another office assistant.
Go to the folder where you saved those two files. Choose an assistant to rename him to, for example Rocky.
Either rename or delete Rocky.acg and Rocky.acs. Now rename Genius.acg to rocky.acg, and genius.acs to rocky.acs.
Now he will show up as an option under choose office assistant.