Disk Cleanup on Windows XP asks the user if they would like to compress all files on drive to save space. While it does what it advertises, it also makes the computer run significantly slower as it has to use the CPU to decompress each file as it is access before it can be used. A lot of files on a modern computer are already highly compressed - think mp3, ogg, avi, dll (a very relevant one to slow computers) - so drive compression doubles up on the work needed to access their contents for no advantage.
