If you have to install from CD, I'd expect that's a totally different operating system install.
The problem with using a skin for XP is that it won't respond the way that it will in Vista. Sure, it might look similar, but since it's not optimized at all, it's just going to look 'ok' sitting still. If you go with a basic skin, it should be fine. Start menu, taskbar, and a few icons, nothing more. You could go further and try to get an Aero-esque program, but I'm going out on a limb to say that it probably won't work, at least not enough to warrant the time to download.
If you *really* want to make the switch, check out
this site. I think I tried their utility a while back in a virtual machine, but I didn't keep it for long. Lagged up the system pretty decently for some of the addons, but the basic skin looked alright. As long as you disable any of the addons they could add (I used one once that had a Longhorn esque (Vista's codename, what Vista should have been) clock, and other features, but they were more bother than they were worth), the basic skin should give you what you are looking for.
--Note-- I haven't checked out that site's downloads, merely the content on the page. Make sure to do a virus scan, and be aware of any new startup programs. I can't vouch for their products.
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