Question: For someone looking after a small 50 Windows PC network (Mainly XP - some Vista), are there any tools that can do an automatic inventory of the network e.g.

* OS
* Memory size
* Hard drive type and size
* Serial numbers
* Monitor type

etc.

At the moment, someone has to go from PC to PC writing all this stuff down.

Answer: 
Assuming you are on an Active Directory domain you can very quickly develop a Windows PowerShell script to do it all remotely via WMI queries. If I could post a link, I would link you to a copy of the script I wrote. 

Apart from writing a script there is the Microsoft Server Center stuff
Our tool is Admin Arsenal. It does require your environment to be managed via Active Directory. www.AdminArsenal.com