The only way you'll have the cd and not a liscense is:
A) You pirated it (you don't sound like one)
B) You SPECIFICALLY bought a CD sans liscense, in which case you'd probably know that you did.
Buying liscenses is only used if you want to install to another computer, or if you already know somebody with the software then you can borrow their disks (and copy them if you want) so long as you purchase a liscense (which makes getting software much easier, especially if you're a student or teacher. a Windows 2000 liscense runs at a measly $54, while Win9x is like $46).