During the setup of Office 2007 you will be asked which of the "old" versions
you wish to keep and which to substitute.
You can also keep all of the old versions and use them side by side with the
new Office altough this CAN create a bit of a hassle.
So there isn't really a need to "delete" (I think you mean: uninstall) the
old Office before installing the new one.
You can however do this if you want to start again from scratch with Office
2007. To do so, enter the Control Panel (Start Menü / Settings / Control
Panel), select "Add or Remove Programs", search for the Office XP (or
whatever) entry and click "Remove" to uninstall it.
Note that this can lead to the loss of the settings you made to the Office
XP programs (such as manual auto-correction entries or changed orthography
settings). But if you don't mind this, you can safely uninstall, if you want.
As I said however, there is not real need to do this.
Sincerely,
Andre