The previous answer is not terribly helpful. The true answer is not really in the computer manual but in the operating system. If you use Windows, go to Control Panel and you will find some form of ystem restore tool - possibly under 'maintenance'. You can also search for 'system restore' in the Windows Help screen.
Your computer automatically creates restore points every day it is used and you should be able to choose which date to go back to. This process will remove any software you have installed after that date but should not affect any files, so anything you have saved will still be there.
If you try to access something afterwards and find you can't open it, you will be able to download or reinstall only the software you actually need. I do however suggest you backup all your files to disk (or onto an external hard drive) before using system restore, just in case.