Hy
Have a look at
http://www.loginter.net/en/loginventory.php
You can either scan all your Windows-Hosts and inventory them via WMI or run
a small executable at every login which reports the inventory to the server.
It has a very easy-to-use interface and you can create user-defined reports.
And in my opinion it's very cheap for it's functionality (300Windows + 300
Devices 2100$)
If that is still too much for your budget have a look at:
http://ocsinventory.sourceforge.net/ I tried it couple of months ago, but
i missed a couple of features.
Hope that helped you a bit.
-Hans