By "sp_whoisactive", we most often mean Adam Machanic's stored procedure:
http://sqlblog.com/files/default.aspx - current version is 11.11.
You can also run sp_who2 and save the output, run it again, and see the difference in disk IO.
Alternately, watch what _time_ it's spiking so much, and run a trace (or Profiler, if you it doesn't hit your performance too much) during that time!