XPe does only ~500Mb and requires a lot of trouble to set up.
No more hassle now!
Please go to http://www.disklessangel.com to find an easy way of booting xp in ram if you have sufficient system memory. It can create a virtual boot-up ramdisk for you to boot full size xp in it up to 4 Gb in size.
Diskless Angel XP also includes many other tools, such as shadow backup of your live xp while running, creating img and dsk image files for virtual machines, and a partition editor which helps you partition your hard disk and easily make bootable USB Flash disk, SD disk, etc.
A demo version is available for download now.