We have 3 10 user licences of KAV which I install on new PCs in the office.

I kept installing the first key, thinking it would tell me when I had reached 10 users as every time you use the key it authenticates it by connecting to Kaspersky's servers (or so it appears)

I got an error saying it was an invalid key on the most recent PC I configured so I thought great 10 must be used up, removed the key and added the second one, all working fine... I thought!

Then, someone comes up to me yesterday saying their serial number was invalid - a PC I configured perhaps a month or so ago on a 2 year licence of this 10 user key, I looked at his PC just now and it looks fine - so is it something connected to 10 users at any one time - and when the users switch their PC on who ever are the last to switch on will get an error?

Is there any way to see the PCs configured for a specific key? - I.e. a front end for someone like me who can enter the key and a password and see the machine names configured to use that key etc?

You can install Administration Kit (no extra license is required) and manage your workstation centralized.
There you can generate reports and monitor your license usage.

http://support.kaspersky.com/ak6mp1?level=2

That sounds useful, how exactly do I set this up?

'Select database server type to connect to administration server' - we have a choice of SQL or mySQL, as we use SQL inhouse I chose SQL but then I was not sure why or how it would use one of our SQL servers so I did not proceed!

I should add that the PCs are not in a domain based envionment, whether this is required for the tool to work or not I do not know...

Just install it using the defaults and it will create a whole new db table for KAV inside your existing SQL server instance.
You don't need a domain either as the Kaspersky Agent will make all communications possible once its been pushed out to your network clients ...

Make sure you install MDAC 2.8 and I like to use Kaspersky's MSDE installer for SQL install, then install Admin Kit last.

My advice for the database server is MS SQL Server 2005 Express Edition (it's for free), a MS domain is not required.
You can install all Adminkit components on a workstation, but remember the limit of ten concurrent SMB connections.