I love Vista, but it is probably not the best option here no. Especially not if you have an older PC.
Unfortunately since you have everything on a single drive that means you will either need to back up all your stuff on a different drive or lose everything if you decide to go for a full reformat/reinstall of Windows. Which I guess is not an option for you at the moment. I can't remember if the Windows installer lets you install directly over an existing Windows installation. If it does then I suppose it's worth a try (you could just pop in the Windows CD and restart your computer and see what the installer offers in terms of options). It would be far from ideal though.
Probably the best thing to do would be to find someone you trust to have a bit more computer know-how than you do and have them do it for you. You will probably save yourself a lot of frustration that way.
There are also some ways to save yourself the trouble of running into these kinds of issues again. Creating a separate partition (i.e. your C: drive) just for Windows and maybe some bare essential programs and absolutely nothing else and putting everything else on another partition (i.e. D
makes it much easier to quickly reinstall Windows without the bother of lots of things to back up (pretty much all you'd possibly need to back up is your user directory). Another thing that I highly recommend if you want to be able to reinstall Windows quickly and easily is to use a program like Norton Ghost. What you do is right after you have done a complete reinstall and have finished setting everything up the way you like it you create what is called a ghost image of your entire C: drive and burn it to DVD for instance. If at some point you want to reinstall Windows you pop in the DVD and let Norton Ghost do its magic and you will have a fresh reinstall done in literally about 2 minutes. There pretty much is no way to make reinstalling your Windows installation any more quick and painless (and easy!) than that. It is the same mechanism that those "rescue discs" you sometimes get with pre-installed computers and laptops use.
But again, unfortunately you'd probably need to find someone who can set things like that up. It's not exactly rocket science or anything but it does require a bit of experience and computer know-how to get this up and running quickly and smoothly, and you don't sound too confident in that respect.