EDIT / THOUGHT: You probably want XP because it does something, like games, 7 doesn't? In general, 7 does everything XP has ever done, right?
Make a new partition or rather, install a new HDD / SSD where you install XP, together with the programs you want. It would be manual, meaning that you have to go to the boot menu to change the order, and you would have to install everything in the XP. But I think, this might be a very safe way to do it - you kill 7, start XP, and counterwise?
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.