the Compaq Presario CQ 56 has a second part to the modle number some thing like CQ 56-101SA
if you could post the full modle you will get a better answer.
if you click on the green box marked drivers and software then enter "Compaq Presario CQ56" it will list all of them
then you need to see if you can match a model up to you system and see if the xp drivers are avalible
if they are you will not have a problem installing xp
if it only lists windows 7 drivers would sugest keeping windows 7
W7 starter is a bit grim you might want to consider an upgrade to W7 home premium 64bit.
do you have a specific need for xp or are you just not getting on with W7.