Alright, after re-re-re-re-reading the OP I think I get the issue now, lol.
You had Windows XP installed, but then you installed Windows Vista. However, after installing Windows Vista, you realised that data from your old OS installation (that is, Windows XP) is still on the hard drive, as it wasn't erased during the installation of Windows Vista.
Okay. Well the likely reason why this has occurred is because the entire
hard drive wasn't formatted prior to the installation of Windows Vista.
If there are a lot of old files on the hard drive left from your old OS installation, and you don't know what or where the data is, you could re-install Windows Vista, ensuring that this time you format the entire drive (and all it's partitions that are no longer required). Alternatively, and perhaps the most convenient solution (if there aren't many old files and you know the whereabouts), is just to erase these files yourself. Getting rid of these files will require some investigation first:
- Do you know where this 'old' data is on the hard drive? For example, is it on the partition where Windows Vista is installed, or is it on a different partition on the hard drive?
- Do you know what this old' data is? For example, is it just system files left from your old Windows XP installation such as 'Documents and Settings' and a 'Windows' folder, or is it System Recovery data for the original Windows XP installation put on the hard drive by the manufacturer?
If it's old system files (e.g. 'Documents and Settings') left over from your old Windows XP installation, you can just delete them. If it's system recovery data (to restore your computer to it's factory state), either on the same or different partition from where Windows Vista is installed, then note that erasing this system recovery data without first having created a set of recovery discs from the data means that you won't be able to restore your computer to it's factory state.