So you have a borked PB laptop and working Tosh laptop. You're asking can you install XP on the PBs HDD and then put it back into the PB, it is possible but a bit of a pain.
Not sure what the best way of doing this is really, you could put the PB HDD into the Tosh and install XP that way, IIRC you can start the XP install then when it goes to restart you can change the hard drive into another machine and it will work.
The trouble is if you do an install all the way through Windows will install the drivers and configure itself for the Tosh then you go and wack it in the PB there is no guarentee it will boot, often it will blue screen.
Im not sure if XP will see the USB hard drive as something it can be installed on but I'm sure there is a way.
P.S never buy a PB