In CRM 2011, you can create your own bulk delete jobs from the UI. Settings -> System -> Data Management -> Bulk Record Deletion. Click New.
Create a filter Look for: System Jobs, Completed On - On or Before - 12/31/2010 and step through the rest of the wizard. You don't need to set it recurring.
For CRM 4 you could look on codeplex and see if you can download a bulk delete tool.
I would recomend making a backup of the org database before doing any deletion. In 4.0 there were recurring system jobs would have to be rebuilt through SDK if they were deleted. So be careful what you delete.
Jason Peterson