Your post tends to suggest that you have attempted to modify your Windows XP installation with one of those theme packs that make XP look and behave (to some extent) as though it was Vista. I did out of curiosity try that once on a spare laptop but to be honest thought the machine was more awful than installing a proper copy of Vista so I turned it back.
In my case this was within a day or two of the experiment and most settings were reverted to their original state by way of Windows System Restore, but to be honest it I still found little quirks every now and again a few months later. Having said that, this was a spare laptop and so doesn't get used much.
If you have had the theme pack installed too long for System Restore to be available then most of these theme packs have an uninstall option and you should try that first. If things still fail after that then you can try an "Upgrade Install" of XP which may restore most of your settings, but you will probably have to re-download all those time consuming Windows Updates again.
Last but not least is a clean re-install of XP.