If you have plenty space on your HDD create a new partition and install 7 there, you will then have dual boot choice 7 or 8.
You other data will not be affected, but backup your essentials just in case...
Assuming they have the same diskmanagement in 8 try this and substitute 8 for 7 and 7 for XP...http://www.howtogeek.com/howto/8790/dual-boot-your-pre-installed-windows-7-computer-with-xp/