If you didn't uninstall Office 2003, it's still present. Have a look at your Program Files folders. Searching for winword.exe, for example, should give both the old and the new one.

If you choose to let the install of Office 2007 uninstall the old Office version, it's gone. It's a clear choice at one of the first install screens.

We don't know what you chose. You do, hopefully.

Kees