Firstly, are you sure that the p4 2.8GHz CPU you got works? Can it be tested in a different computer?
and secondly, what motherboard (or make / model of PC) do you have?
There could be any number of things causing it, but in all likelyhood it will be one of three:
1. The CPU doesn't work
2. It is not compatible with your motherboard
3. There is a motherboard jumper setting or BIOS adjustment that needs making