Its it possible to buy a Bootable External HDD? Or do you have to buy software to make your own?

You can boot off of an external hard drive, but your BIOS must support booting from the USB or Firewire connection--that's the key

But can you buy a bootable HDD? or you just buy a regular external and install an OS?

Weather you have an external hard drive or an internal hard drive it is generally the SAME DRIVE. The difference is when it is outside it talks to the computer using USB or Fire wire connections. You can make the hard drive bootable but if your computers BIOS does not support booting from USB it will not work. What do you mean buy a bootable hard drive? Do you want to purchase a drive with XP-Pre-installed on it? That would be hard to do to configure it to your system specs. Somebody asked me yesterday actually, they wanted to go to Best Buy and purchase a hard drive for his computer with XP already installed. I said they do not just sell drives like that because the same drive would have to be set-up differently if it was a Pentium 2 machine then if you had like an amd 64 bit machine

Most systems nowadays support booting from USB, so YES, you can buy a bootable removable HDD. My suggestion, however, is to buy an internal HDD and an external enclosure for it; its usually MUCH cheaper.

PS: Whether, not weather.