Straight question.

How to install windows xp in a system which is running windows 7. (without formatting windows 7).
DUAL boot

there are various ways but probably the easiest is to use a reasonably good disk partition program. you will need to put a new partition on your hard disk (beware not to rub out the old one with 7 on it)
Then format the new partition and install xp
then use a boot manager to let the hard disk enable you to chose which OS.(or buy the upgrace to professional 7 that includes the xp emulator
if stuck look up these words above on google.