Before spending BIG bucks on Rotors, there are some other tweaks to try first. Wider 'Q' factor bottom bracket, spreads your knees wider apart, usually indicated by pain on the outside of the knee. As mentioned spinning is good, as is closer seat adjustment. Counterintuitively adjusting the seat back angle can help (less mashing laying down?). Also shorter length crank arms have helped some people, 175mm is common, 170 or 165mm are reasonable substitutes. One friend of mine found out she has different length legs, so put different length cranks on her bike. That worked for knee pain on only one side. Fixed a hip zinger she would occationally get as well.
Darren