I frequently fire up utorrent while using 2 miners in GUIminer. I haven't seen a performance issue at all.
I also use news groups and when downloading from an nzb I have to throttle it a bit but that's a whole different monster.
It's the exact opposite for me. I run two miners as well but I have to turn off the first one while I'm using this PC for anything other than just mining, or else it will freeze up. Granted, both of my GPUs are overclocked 30%, so that may be the reason for the freeze.
The uTorrent problem may have to do with the 100% CPU usage. If you have a multicore CPU try limiting your miner to only some of them. There is a checkbox in GUI miner.
As for freezing up, simply choose a less aggressive OC on the card that feeds your monitor. If you account for all the downtime, this will actually make you more bitcoins. There is also a particular problem with Flash player while mining, search the forum for the cure.