Uninstalling Windows in External HDD
 

I just bought an enclosure for my old HDD, but it has Windows XP. Is there a way to remove Windows without reformatting the drive?

How about deleting \Windows, \Program Files, and all system files at the root (config, hiberfil, io, msdos, ntldr, pagefile, ntdetect, and boot.ini)...?

Backing up is nice too.

If it were me I would just backup my data and format the drive. You will probably find a fresh drive with only your needed files on it will perform a bit better through the connection media due to less latency in seek times.