THINK DATA: Data that is proprietary to your company and cannot be replaced, eg.

E-Mail, Databases (SQL, Exchange, etc), Customer Records, Invoices & other Financial Data, Presentations, Contracts, Pictures, Drawings, Your Bookmarks, Your Desktop, Your Appointments, Contacts, To Do list in Outlook, Files you've synchronized from your cell phone to your PC, Your My Documents folder.

What you shouldn't need to back up?

Anything that's easily replaceable. Installed software typically your Operating system and  applications are replaceable. They're available on a CD or downloadable. With the license keys, they can be up and running quickly.  You do not need to spend time, effort and money backing this information up. It is still a good idea to store all your license keys in a simple text/xls file and back that up as well.