Can a Dual-Core processor both focus on a single program?

Ok, so I've recently got into PC gaming. My PC has a Intel Centrino Duo processor as 1.8GHz each. There are many games which list me as being able to play them PAST the recommended system requirements (according to srtest.com). Such games include Bioshock, Tom Clancy's Rainbow Six: Vegas 2 and others. But when I play such games even on the lowest setting, it is rare that they do not lag. I don't know why because I have no other program running in the background so I'm curious if it is because my dual-core is only having one processor running the game. I plan on getting M Effect for PC soon and I also meet the recommended requirements for that too. But I fear that it will also run slowly regardless, is it possible to have the program take advantage of BOTH processors? (BTW, I have Vista Home Premium)