Try the following troubleshooting steps and see if this fixes the issue.
1. Check if the problem is only with one particular application or its happening always.
2. Start Windows in Safe Mode by hitting F8 during startup and then select Safe Mode. Test the keyboard in Notepad in Safe Mode.
3. If the keyboard works fine in other application as well as in Safe Mode then the problem is not with the keyboard but some software is messing it, probably a virus, try doing a complete scan of your computer using any Antivirus software.
4. If the keyboard is not working fine both in Safe Mode as well as in other applications, this might be a problem with the keyboard itself. Try cleaning the keyboard. Also see if any of the numeric keys keep pressing while not in use.
5. If the above step doesn't fix the problem, use a different keyboard. If the other keyboard works fine, replace the keyboard.