Yes - your brain ;-) Seriously, if you mean accumulated "bloat" which arrives slowly and rather secretly with the normal operation of things, then CCleaner will get rid of it for you. It will even get rid of unnecessary registry entries [back up what you repair]. click here Google "duplicate file remover", lots to read and choose from. If you are referring to unused programs then only you can decide what to get rid of. Do a full backup before you begin: Start > Programs > Accessories > System Tools > Backup. If you are running XP and you don't see "Backup" you will require your XP installation disc to install the facility: click here Create restore points before you remove anything you are unsure about: Start > Programs > Accessories > System Tools > System Restore > Create a Restore Point. Have fun.
<Start><Settings><Control Panel><Add/remove programs> will remove most software including registry settings, but you may also have to remove the folder from your drive afterwards. For future use, I would recommend you stored all your 'experimental' software in its own folder or, better still on its own virtual drive, then you can see at a glance what's where.