You will either install Windows 7 to a separate partition or will wipe out the previous Windows version during Setup.
If it's the latter, please----be sure to backup all your data first. A clean install with Upgrade media is just what it sounds like: You will perform a clean install of Windows 7 using an Upgrade version of Windows 7, instead of the so-called (and more expensive) "Full" version.
For some people, it just works, and I'll provide information here so you can help ensure it just works for you, every time. For purposes of this discussion, a clean install--or what Microsoft calls a custom install--is when you boot your PC with Windows 7 Setup media (typically a Setup DVD, but with this version it could also be a specially created, bootable USB memory device containing the Setup bits) with the intention of installing just Windows 7 on the PC.
There could be a previous version of Windows (XP or Vista) installed on the PC already.
However you do it, these tips, tricks, and workarounds should get you up and running in Windows 7 legally, safely, and easily.