A safe way is to not use an entire song/track.
Copyright has what's called a fair use policy so you see TV do this a lot where they take snippets of music and put it through.
Can be done easy enough and perhaps should be done as you will be using your video to promote a product, sales and income.
So what's fair use? Enough that you don't get busted if challenged. There's no fixed rule. It's open to interpretation and what the original copyright holder thinks. If asked to remove it you should. Fair use could be 10 seconds or 30 seconds ... who knows.
Also running credits back to the author/copyright holder wont help.
You shoot the video, overlay the talking you need to do then slide in the music. You can purchase for minimal cost certain tracks ... search around.
With Ulead10 there's a set of tracks that are allowed and paid for, they call them licensed and you can buy more of them. Also it has the great feature of making a track perfectly fit your video in length with the right fade out ... all automatic.
Check out this website, it's where I can buy more licenced tracks automatically via Uleads download and update feature.
http://www.smartsound.com/index.html There's a whole royalty free section here. You can preview them and they'll be able to automatically be sized right for Ulead 10! But I just run with the std ones they gave me.
http://www.smartsound.com/products/