I understand you said you wanted a desktop version. But for many reasons I would strongly suggest using a web service.
1) Your files will be permanently backed up meaning you never have to worry about
2) The learning curve will be quite easy as they are built to be intuitive
3) They will generate much better graphs and allow you to absorb more information faster then you could initially imagine
4) You can access your data from anywhere at anytime
5) If your worried about security, I would say that modern day web apps are possibly safer to storing a flat file on your computer as anyone can hijack the file or retrieve it off your usb stick.
Now I have a semi-strange suggestion in mind for your needs and that would be Freshbooks.
http://www.freshbooks.com/clientsandstaff.php
It's actually built for invoicing but it will cater for exactly what you need.
You can assign as many projects to a client as you like and easily see how many un/finished projects there are.
If you have never used a web application before it might be a bit of a learning curve. I strongly suggest learning as you will find your time dramatically saved, your potential increased and will feel more godlike in your business as you have a birds eye view of aggregated data available real time which is a huge step ahead of your competitors who have a few numbers in excel.
Goodluck! Thomas
p.s. Checkout the Tour to see EVERYthing else that comes with it
Client/Project Spreadsheet 
