As far as I am aware, none of the Microsoft OSes will allow you to roll back to a previous version of Windows after an upgrade.
If you do a clean installation of an OS (vs. overwriting the previous version)you might be able to resurrect the previous version (if it hasn't been removed). An upgrade is installed in the same directory as the previous version and overwrites numerous files as well as the registry. The old versions of files and the registry are not saved.
XP installation upgrades the system files and in most cases the NTFS directory versions. Some of these changes are irrevesible. It is never a good idea to think you can undo such a significant change. If there is a chance you would be forced to undoa system upgrade then you should have a tested plan in place to do a full restore of the system, prior to the upgrade.
If you're asking for technical help, please be sure to include all your system info, including operating system, model number, and any other specifics related to the problem. Also please exercise your best judgment when posting in the forums--revealing personal information such as your e-mail address, telephone number, and address is not recommended.
XP installation