First off you shouldn't be doing both cardio and lifting weights on the same day... talk about burning muscle
. Aside from that, if you don't really care about losing muscle then make sure to do your cardio AFTER lifting weights. If you do it the opposite way, your body will not be able to expend enough energy to even make the weight workout beneficial. Kurlblind, you are correct in hearing it is good to have a 5 minute light warmup before lifting weights. It helps warm and stretch your muscles and highly reduces your risk for injury while lifting.