There is a program called Tweak UI which can be downloaded and installed for FREE. I don't have Vista but seem to remember there being an option in there to Prevent Programs from stealing Focus (or words to that effect) You run Tweak UI from the Start Menu and click the General Tab in left column (to expand list) then click Focus - you should then find the box to un-check "prevent programs from stealing focus"
There's also some other interesting features available in the program
Download from here Tweak UI
Hope it works for you.