Ways to prevent yellowing have been suggested, but if it is already yellowed, you can attempt to reverse the yellowing by completely taking the system apart, and scrubbing the yellowed parts lightly with a blend of water and a little bleach. Technically, you are scrubbing the very top layer off, while bleaching it, so if you want 'mint' condition, you're out of luck. But it does remove the yellowing. (And since the yellow is a molecular change to the base material, there is no way to truly "reverse" yellowing in the material.)

As for dual boot, yes, it's easy. You use two partitions. There is a piece of software that lets you dual boot from a single partition, but its name escapes meat the moment.