Spiceworks - Free! Inventory, Helpdesk, Reports...
Just thought I'd point out some software I was told about at a recent meeting. I've not had a massive look yet but on the surface it seems a nice piece of free software.
It does an automatic inventory of hardware and software on the network, will alert you when some things aren't up do date ie Sophos, alerts on low disk space, all sorts.
It also has a built in helpdesk, though I've not tried it yet.
Anyway, just thought it might be of some interest.