You can run SETI@home hidden by running a little prgram called, oddly
enough, "SETI Hide".  You can find it easily enough through a link from the
SETI@home website.

Now as to whether or not you should to run it without your employer's
permission, that's another story.  Is running S@h on your employer's PC
without authorization really worth the very real risk of getting fired,
fined, jailed,  sued, and/or all of the above?