I assume that your hot water line is leaking and you plan on capping it off and using the existing cold water line as your new supply? What happens 6 months from now when you get a leak in that cold water line? Maybe you should reconsider, repair the pipe, and work on trying to prevent leaks in the future. After some research I have come to the conclusion that there is two viable options to prevent the further deterioation of your piping system.
1. Install a chemical feeder, available at http://www.aquasmart.com. I had a similar treatment system installed about 2.5 years ago and it seems to be doing the job.
2. Try using a cathodic protection device. http://www.copperknight.com. I have not installed one but the theory is viable.
All this assumes that your deterioation is occuring from the inside out, but you won’t know that till you dig out the pipe.