Work Order/HelpDesk Software

you should look at getting some professional server based software as opposed to something that works with M$ anything. No matter what has been said, NOTHING integrates easily with Outlook (or any other Office product for that matter).
TigerPaw has some excellent invoice/work order solutions, but you'll probably end up looking at Unix applications for stability and security, I think they'll hang up if you even mention Outlook junk to them.
Try to avoid the Microsoft mess at all costs, you'llfind a much more stable system that doesn't have limitations.

have you looked at Remedy. It does not interface with Outlook and office though.

Another might be just busing the functions in Outlook task management?

Its a mmatter of how much control is wanted over the work flow. Outlook provides little or none of course. It just coordinates.