Christine,
It is possible to see Lake Como as a day trip frim Milan, but it would be much better to try and find an extra day so that you can spend a couple of days there.
As Kent mentioned, Varenna is a much better choice than Como. It's a beautiful small town and provides easy access to the Ferries for access to Bellagio, Mennaggio or other spots.
Again as mentioned, be sure to read the description in Rick's book on travel to Varenna. The train only stops for one minute, so be ready!
This is one location where I strongly agree with Rick - once you're there, you'll be tempted to throw your Itinerary in the lake! I can hardly wait to return!
One question though - are you spending any time in Milan? My preference would be less time in Milan and more time on Lago di Como.
Happy travels!