I did this with a computer that had a xp os originally and installed a win 7 into a new empty drive. In fact, xp install software can automatically select where you want to install your os to, so you dont need to take out your other drive, just put the xp disc in and select for the drive that you want to install it on, after you are done, if the computer does not ask you for the orignal xp os, configure the boot file in xp: boot.ini in the root of the drive:
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows XP Professional" /fastdetect
multi(0)disk(1)rdisk(0)partition(1)\WINNT="Windows XP Home" /fastdetect
Something like this, you need to search for boot.ini configuration or such softwares.