get the instrumental and use the phase inversion technique (use google to find how to do it exactly).
or
download adobe audition. open the file, and theres this filter called "center channel extract", use that filter,and choose acapella. it doesnt work for all files but try them and see. it wont make a perfect acapella,you will still have the kicksdrums and snares since they are mostly on the center channel.
You can get a 30 day trial from the adobe website
www.adobe.com,then get a crack.
here's a 30 sec clip of street fame. i use the adobe audtion method.
http://s7.yousendit.com/d.aspx?id=18MJD9TSSDV4Q31RL6K8NIAQMB