Try: Menu, Settings, Applications, Manage Applications, YouTube, Clear Data and Clear Cache.
Also Try: Shut down phone, pull battery, wait 30 seconds, reinstall battery.
Important Note: If you are using any Task Killers, disable or uninstall them until they are properly configured.
Resetting you entire phone is the last resort and you will loose all settings and apps will need to be reinstalled. Not Recommend unless necessary.