For a legitimate install CD you can choose to back up your old Operating System, then revert back to it later if desired. However, I would not count on anything with an illegal copy, plus you may run into problems with activating it or in installing necessary updates. If you think it is bootleg it probably is. Do you really want to go down that road?
If you performed an upgrade from Win98/Me, elected to backup the
old system files, and didn't convert the partition to NTFS, then all
you need do is boot into Safe Mode and Start > Control Panel >
Add/Remove Programs. All of these conditions _must_ be met for the
uninstall option to be available.