running Hadoop software on office computers (when they are idle)
You can use task scheduler to detect idle state and then start/stop a hadoop vm with virtual box or vmplayer. Or you can write a powershell script that does start stop based on resource usage.
Hadoop is not a computation grid it is a more a data grid (see slide 9 in this presentation). The point is that with hadoop that data is spread over the cluster and thus the data has to be stored on the computers. The time it would take to copy the data over/remove it when they're not idle would probably not be worth it - you'd be better off using hadoop in the cloud (amazon,Azure etc.)