OK locate the files (generally in the users directory) that you want to keep, pictures, documents etc… Also make a note of all licences and passwords.
There is a guide here http://www.geekstogo.com/forum/topic/173729-reformat-and-install-of-windows/ although it is for XP the basics are the same
From the Reatogo desktop you should be able to move all the data to a USB drive
What is the make and model of your computer ?