Microsoft has just launched a new tool that plugs into Excel that allows you, not only to count how many people are searching for your brand (or your competitors') on Windows Live Search, but also to learn more about them.
It uses your Microsoft Live ID (used to be called a Microsoft passport) to add demographic variables. Everyone that has a hotmail, or MSN Messenger or Windows Live Spaces account, will have a Live ID, so the sample is pretty representative.
As the number of users is lower than Google, then i would recommend using Google to define absolute numbers, but if you are a big enough brand to show up on the stats in the Microsoft tool, then this a very useful free tool.
The neat thing is that as it is an Excel plug in (and is free) then you can take the data and manipulate it, graph it, rank it, combine it etc.
Why not check it out for yourself (proving you have Office 2007 as it doesn't work on previous editions - but you can download a free trial copy of 2007 on the site).
It has a vanity URL and is called "let search make you smarter" at the moment.