Linux boot thingy

[pmeswani]

That sounds horribly like it might break everything.

Thanks for the replies. I don't recall any other options in the install, but I probably wasn't paying enough attention.

If you are worried about breaking the Linux Boot up, get a knoppix (or any other live CD) and boot up the system with it and change give the primary drive an MBR. Thereby, you will have 2 Bootable drives. At the end of the day, the primary drive is missing a MBR tag.

[PingPing]

Went back to boot from Windows and I now have the Linux boot menu. So it appears it's installed something to my primary hard disk without me asking it to.

It's installed GRUB on the MBR.

why did it get me into this mess and how do I get out of it?

I presume you're using the Ubuntu LiveCD as Ubuntu is where most linux newbies start. Please correct me if this assumption is wrong.
During the installation of Ubuntu from the LiveCD there is an option for you to specify where you want GRUB to be installed (default is the MBR). The problem is that this option is not made plain to the user and you have to hunt for it to find it. It's there, but basically you have to know what you're looking for in order to find it. IMHO this is a design fault with the Ubuntu LiveCD Installer application and not a problem with linux (ie. computers will only do what they're told to do).
To fix your problem I suggest 'you wind back the clock' with respect to the GRUB/MBR problem by reinstalling Windows' bootloader, NTLDR. To do this, boot you PC from the Windows XP install CD into 'rescue' mode (not install mode). Once you're in rescue mode run the 'fixmbr' command at the command prompt. This will reinstall NTLDR on the MBR. Then reboot as per normal and you should be back to your normal Windows experience.

Let me know if you still want to persist with installing linux to your USB drive and I'll help you out with it. But please confirm what distro you're trying to install (Ubuntu, SuSE, Knoppix, etc.).