Off the top you could look at this free program If you are on the
windows platform.

http://www.whatsrunning.net

Just tried to link to is but it must be down.

You might want to manage that at the operating system level, which
already has a task scheduler, rather than trying to implement one
yourself.  That is, launch the JVM that runs your file searcher using a
priority below that of normal processes so that it will essentially grab
idle CPU time but not much else.

Use task scheduler to schedule your task. In Settings for that task
check "Only start the task if computer has been idle for at least x
minutes".

Tethys Solutions, Expert Services Group
http://www.tethyssolutions.com/ask-the-expert.htm
SMART Macro & Automation Software