Anyway I'm assuming its a P4. You should first have checked to see if your motherboard needed a BIOS update to support a faster CPU, or if it was capable of supporting that CPU at all. Also, there are at least two 2.8Ghz P4s, one with a 400MHz FSB, and one with an 800MHz FSB and Hyper Threading.
Why? Sir, do you have any knowledge about computer hardware? As far as I know, he has an outdated I9 which is hardly upgradeable, but he can change only the processor inside it (like the title suggests) to check if it will work. It is worth a try.