You don't necessarily have to download XP Mode, unless your going to run it within Windows 7 Virtual Mode.
You don't have to kill Windows 7 either.
If you want Windows 7 and XP, then Titanics suggestion of installing another HDD is the way to go.
Install the new drive, disconnect your Windows 7 drive, install XP on the new drive, once you have XP up and running with no issues and all Service Packs & updates installed, shut the PC down, reconnect your Windows 7 drive, boot up to Windows 7, install EasyBCD, also mentioned by Titanic, and add XP to the boot menu, now the next time you turn your PC on you will have a choice of XP or 7 to boot to.
Hope this helps
Don