I have had my Zen Micro for a while now, and i have noticed it getting slower and slower as time goes by. this is mainly because of the podcasts i use it with. Every week i load about 150mb of podcasts (maybe more) and every other day, previously listened to ones are removed. leaving chunks of days split all over the place. Well, there is a tip i found over here telling you how to get to the recovery menu:

There’s a hidden ‘Recovery’ menu that lets you defragment or format the hard drive, reboot, or reload the firmware. This is for advanced use only, and you’re advised not to play around with these options unless you know what you’re doing.
To access this, take out the battery, hold the switch in the ‘on’ position, and with this held to ‘on’, reinsert the battery. A menu will appear.

Handy for thouse of you who have a Zen. Clean up, i think, is defrag, fromat is what it says on the tin, and same with the rest. handy!