Are you using Windows XP?
It is really on safe to move your own data files - documents, photos, videos and music. If you want to move programs it is far safer to delete the program in the add/remove this program facility and re-install it on another drive. This means you must either have the original cd or be able to download it.
Are you familiar with Windows Explorer? If not, read this and the next three pages at least:
http://www.homeandlearn.co.uk/BC/bcs1p6.html
That will help you safely move your data files.
You probably have a lot of programs on your hard drive that you never use. If so, remove them - Start - Control Panel - Add or Remove Programs. Be sure that you don't want the program before you delete them.
You should also clean up your hard drive - Start - Control Panel - Perfomance and Maintenance - Free up space on hard drive.