BOAC: I've never yet succeeded in getting my laptop (old one or new one) to boot from a hard drive on a USB port. In the end, to preserve XP capability while installing a new drive to run Win 7 Beta, I bought a caddy that goes in the CD/DVD drive slot. That boots just fine! If you have a machine with a suitable slot, that's the way to go. Don't bash your brains out trying to get an external HD to boot: I don't think you'll win.
Aside from the opinions about Linux (I have a separate machine here to try out Linux flavours, but no beard, no pipe, and no red jumper. I do have some black shirts, if that helps)...
Some Linux distros are startlingly competent, and a lot faster on any given hardware than Windows. I note that the three ISPs I use for different purposes all have Linux running their servers. It can't be that bad.
I got on well with it - with most of the popular distros on different partitions on the several hard drives in the test PC. Yes, there was a learning curve, and someone brought up on Windows would need time to learn the differences. But once that's done, they have a free operating system (none of your £179 "upgrade" bills), vast amounts of software available free, and the opportunity to wear beard and sandals if that's their thing. I was close to dropping Windows and becoming a Linux person.
Where Linux and I came apart was when the graphics card in the Linux box popped its clogs. It was a very old AGP card. I replaced it with a different model of card, whereupon all but one of the Linuxes threw hissy fits and refused to start the GUI. It needed a new driver compiling into the kernel (I might have got the words wrong there, and the purists might come after me), and I don't know the relevant series of commands to do the job. Of course, anyone with more than a couple of brain cells would know the correct sequence of ./configure -this +that /the other but I'm a thick linguist and theologian, and pooterspeak isn't my first, second or third language. I gave up.
At that moment, along came Win 7 Beta, faster than XP, without the extreme oddities of Vista. That's more my style.