What Golf Balls should I use if I have a slower swing speed?

For slower head speeds, you should use softer balls with lower compression. The 2 that work well for me are:

Nike Power Distance Power soft golf ball 

This is the ball I use. I get at least twenty yards more with this ball vs. other balls.

Pinacle Gold FX Soft 

This is cheaper than the Nike but equally effective.

Fore!
Look for balls that have a compression of 80 or less. The slower yourswing, the lower the compression should be. The "Noodle" golf balls arepopular with some of the seniors I golf with.
Golfballs targeted for women can often have much lower compression andmight work well if you can get past the macho thing.

Also something like the Precept Laddie or the Wilson 50 would be perfect for you. Choose a 2 piece, low compression ball.