Bad idea, if I mine my computer is very slow, I have to stop my other programs.
At the moment I'm doing some programming in Visual C# 2010 Express, I've got a few tabs open in Notepad++, I'm seeding a few torrents in uTorrent, I'm listening to some Pink Floyd in foobar2000, got my namecoin gui running and the namecoin deamon, and Firefox with a few tabs open. I'm still getting around 330 Mh/s with no slow down of my computer.
I have noticed the slowdown your talking about though. I get it now and again when I visit certain websites, github for example. I just temporarily turn on on my workers off though and when I'm finished with that website, restart the broser and turn my second worker back on. Problem sorted. I can even watch video without any problem and still get 315 Mh/s.
You just have to configure your system properly.