Most laptops don't ship with CDs anymore. They have a program that will burn restore CDs. If you haven't deleted that, it may still be there, you can burn OS/Restore CDs.
Some laptops come with a restore partition instead. Honestly not sure how that works.
Otherwise visit the manufacturer's support website, lookup the laptop model and download/burn the drivers you will need (video/sound/network/etc) and do a clean install from any generic operating system disc.
just a word of advice... old PCs run slow not only b/c of data.. but also b/c harddrives begin to fail. hard drives die slowly. they begin to run slower and make noises until they ultimatley konk out.