installing Ubuntu on an external harddisk bootable from both a PC and a apple macbook

[Lamasu]

Hello! I am new here and I have the following problem:

I would like to install Ubuntu onto an external HD, but then I would like to be able to use this HD tu run Ubuntu on both a PC (with win XP on the internal HD) and on a apple macbook (with snowleopard on the internal HD).

First of all: is it possible to do something like that?

And if yes, what have I to pay attention to, at the moment of the installation?

I would like to install ubuntu from the PC (the CD reader of the apple doesn't work). Could this generate any problem if I will use it on the apple?

Thanks in advance!

[Lamasu]

and also, can I use the windows intaller to do that or have I to install id from a CD?