Do your client's computers have modern dedicated graphics cards installed? ATI 58XX , 6XXX, or 7XXX series? If not, it is not worth your trouble. You'll make pennies a day while your client pays dollars a day for electricity. If so, it may be worth your trouble, but only for a few weeks, until ASICs ship out. Once ASICs are up and running, graphics cards will be just as old and slow as CPU's are now, and you'll again be back to making pennies a day while your client pays dollars a day for electricity.
I just looked into ASICs. This is the link I just read:
http://bitcoinmagazine.net/butterfly-labs-releases-more-asic-photos/Should I hold off mining using client's computers and instead buy one of these?