Hello and elcome to TYom's Hardware Forums.
I'm not exactly clear on what you want to do but you're thinking of taking Windows 7 out and installing XP, there are more problems than just not having a CD ROM. External USB powered CD ROMs are available quite cheaply.
First off, I recommend that you look at the manufacturer's specification first to check that drivers for your hardware are available to work in XP. If they are specific to Windows 7 you might find some things won't work and it's too late to go back.