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.
I want to uninstall my Office 2007 trial version and go back to my old 2003 version. BUT I DON'T have my old 2003 disks or my old product key number!!!
What do I do??? Or have I permanently gotten screwed out of having my old 2003 Office because I took the bait and tried 2007?