Here are some useful lan inventory tools:
1. I used this tool: http://ondemand.manageengine.com/service-desk/
It was flexible and offered both a helpdesk trouble ticket solution as
well as the hardware/software inventory and is hosted, so, it didn't
require any on-site installation except for one agent to funnel the
inventory results through.
2. We use LanSweeper. Highly customizable to the point of adding nearly any
tool to the CMD bar for devices. Fast and efficient. You can use the
FREE version or buy as the cost is low. No agents to install. After
initial run, you'll want to start telling LanSweeper what certain
devices are. I really like the grouping options and the ability to add
any tools you can possibly run on a device remotely.
3. Spiceworks http://www.spiceworks.com/.