My friend bought a copy of Win XP Pro and he has a blank drive to install on. Is it possble to install this separate copy of windows through my Windows and setting his drive as a slave drive? I'd rather not go through the hassle of setting his up as primary drive and installing from a cd boot, as it gave me an error last time.

If you mean that you want to install Windows while the drive is in your system, and then put the drive back in his system, no- that won't work, or at least not gracefully. For one thing, unless your systems are identical, you'll run into hardware/driver conflicts when XP boots on your friend's machine, because he'll have different hardware for which Windows was not configured. The other problem is XP's anti-piracy activation/validation scheme, which will automatically sense that your friend's machine is not the machine on which Windows was originally installed. To get XP to run on his machine, you'll probably have to call Microsoft and go through the hassle of getting a new activation key.