Windows 7 doesn't offer the possibility to downgrade like Windows 10 does in the timeframe of a month. You will need an original Windows XP installation disc to restore the system and the C: drive needs to be reformatted, because you can't keep your files and programs from Windows 7.
Although the support for XP has been ended, Microsoft still offers users the possibility to read how to perform a clean install of the operating system. Access the following page to get the instructions: https://support.microsoft.com/en-us/kb/978307