This is probably a slightly more complex question than you think… and depends on what you mean by USB installation
??
If you mean create a LiveUSB memory stick (pendrive)… NO it won’t make ANY changes to your hard drive AT ALL… BUT, not all PC’s can boot from USB memory sticks, though I’d take an educated guess that if your system came with Vista pre-installed, it’s probably new enough that it can.
If you mean, install it to an external USB hard drive (and Windows is on the internal drive)… again NO, though depending on how you set it up it may need to overwrite the Windows bootloader, but you will still be able to boot Windows.
If however you mean install it TO the internal hard drive FROM a USB stick/drive, then it depends on what type of installation you plan… you can:-
a) Resize the Windows partition, leaving room for Linux native partition(s), and install it to that… Linux would then be able to access files on the Windows partition, but Windows will not be able to see the Linux partition
(Be Aware, any partition table manipulation carries a small risk of corruption, so backing up any data you can’t afford to loose would be a good idea)
b) Install Ubuntu INSIDE Windows (kind of as an application, though that is an over simplification, it’s actually a virtual drive) using the WUBI installer, and if you don’t like it you can uninstall it from the Windows Add/Remove Programs control panel applet… there are however pitfalls to this kind of installation… you cannot resize the virtual drive, windows viruses may be able to damage the virtual drive file, and if you decide to fully switch to Linux and delete your Windows partition, well deleting Windows will also kill Linux, requiring a full reinstall.
So the question must be… what do you mean by USB installation ?
I have a large music collection & personal documents, & my hard drive is full, with nowhere to transfer them to in the meantime.
Well (if installing to the same hard drive as Windows) this boils down to how safe do you want to be ;)
I also have 2 (also full) x-drives on NTFS- will they work ok or will they need configuring?
What do you mean by X-Drives… Windows Azure drives ? … AFAIK (and I could be wrong) Azure drives are Windows virtual drives, they can be mounted as network drives, but require Windows to be running to mount them first… so AFAIK they aren’t going to work in Linux.
Will the Linux installation strip & replace windows?
See above… it depends on how you install it, but only if you tell it to.
And while I'm here - when I click the "try Linux" link on the home page of this site, it takes me to an Ubuntu download - I don't get it. Why isn't it a Linux download?
Hmm… I’m unaware of any “Try Linux” link on this sites homepage (or the Linux.co.uk homepage), but that said Ubuntu is the leading Linux distribution, though there are MANY others.
A bit of an explanation -
A Linux DISTRIBUTION (or distro) is a collection of 3 main parts, the Linux kernel, the GNU toolchain, and a collection of applications.
Linux Kernel = Think of it as the core of the operating system, the part that directly controls the hardware, it’s a bridge between applications and the actual data processing done at the hardware level.
GNU Toolchain = A collection of programming tools, utilities, and libraries necessary for applications to be built and run
Applications = This is mainly what changes from distro to distro… the applications included, and the desktop configuration etc.
So “Linux” has come to be used as a blanket term to mean any distribution that includes the Linux kernel & GNU toolchain… Ubuntu ATM being the most widelspread, and probably the easiest for new users, if for no other reason than they’ll find help/support easier to find 
But if you want to get an idea of just how many Linux distributions are out there, see here:
and that isn’t a full list :o
If you have any other questions feel free to ask… and I’ll try not to overload you next time 