There are three kinds of files in your computers...
Operating system like windows or linux
Application like MS Office (excel, word..etc) or firefox
Data like MS Office documents or application database.
It's easier to reinstall the operating system and the application because :
1. You would want a new more powerful one anyway
2. Even when you wanna stick to the old one they are pretty much standard.
3. Transfering them without reinstallation require advance knowledge of IT technology.
What you can do is move your data to external storage like (flashdisk (easy but small, avg cost), CDs (hard, small but reliable and cheap) or external harddsik (easy, huge but expensive).
Data which is independent like office documents are safe to copy but data that is highly related to the appplication like database will require a process called backup and restore.