There is no BIOS update listed for that PC either, which would enable the processor to be recognisd by it.
Unfortunately, Compaq PCs are usually very limited for upgrade potential. The BIOS is generally crippled somewhat, and does not accept substantial processor upgrades.
That said, it may be a motherboard chipset limitation. The motherboard may be able to accept an earlier Pentium 4 2.8GHz version, but not the latest model which runs at that clockspeed. If that is the case then nothing will help. But before you give up try removing the motherboard CMOS battery for several minutes at least, then replacing the battery, power cord and power and then booting it up.
That procedure 'resets' the system, ans it may then recognise the processor. Worth a try, anyway.