External harddrive install (Ubuntu)

Ok so I'm sick of windows (who isn't) and I really like ubuntu. I even have a persistent install on a 2GB flashdrive. Let's face it, 1GB isn't enough space. We have a 500GB seagate external harddrive from when I got a virus on our computer. ( I don't understand why it's 500GB, or computer has barely more than 200) So I might as well put it to use. My dad has admin rights on vista and a firewall so I can't do ANYTHING. A 15 year old tech savvy kid can't live like that. I'm currently learning c++ and I need to experiment with things.

So I would be installing it right now but I have a question. I have seen that you take out the internal harddrive as a precaution. Do I NEED to do that. If I don't HAVE too, then I really don't want to. Is if really necessary? If I don't take it out what are the problems that can arise? What possibly can happen to the MBR?

Like I said I'm savvy in this kind of stuff and I know what to do, I just need reassurance. Thanks for your help.

Oh by the way it's lucid lynx and I have an HP pavilion desktop.
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(I posted the same thread on the ubuntu forum. No replies >.<, hoping you guys can help.)

Well if you're using the standard graphical installer I don't think there is any way you can change how grub installs, but with the alternative one there should be an option for advanced package management or something. So if you're using the default installer I would take out (or better yet just disable) the internal HDD just in case.

Alternatively if you don't want to fiddle around with the internal organs of your PC, just restore the windows bootloader and reinstall the grub bootloader on the external disk after you have successfully installed ubuntu (if you're lucky it might not even install grub on the main HDD).

If you manage to mess things up just google SuperGrubCD, it's a life saver when you need to sort out bootloaders & stuff.

And one more thing - what's wrong with having grub as the bootloader? /