Well if you are on the stock ROM and not rooted then you cannot get into the recovery and clear the cache and dalvik because you have to be rooted and load a custom recovery on the phone in order to access that special menu, and that's where you load custom ROM's from and other items that you want to flash, below is a link to a thread about all the great stuff with rooting, flashing custom ROM's, see below link:
http://forum.ppcgeeks.com/htc-evo-4g...roms-wipe.html