Which programs, folders, and files should be encrypted?

I am going to use truecrypt to encrypt my usb drive. However, I will occasionally be working on computers what I won't have admin rights for. On these machines, I will not be able to access anything that has been encrypted. Therefore, I think that I don't necessarily want to encrypt the whole usb drive.

What programs, folders, files should be encrypted? For example, I don't think I need to encrypt the firefox application, but I would want to encrypt the files with the bookmark and password info.

Thanks for you halp!