If you are having problems, updating to a different operating system may not sort these problems, it rather depends what problems you are having. If you have an old PC and your problems are software issues, just format and reinstall Windows 98.
If you still want to upgrade, restart Pc with Windows XP disc in CD Rom and follow on screen instructions, If you are not using an authentic Windows XP CD you will only be able to use it for 30 days, and it is difficult to get around not having an authentic serial number.