I have a 2.5 yr old computer with pentium 4 processor. is new i-series processor compatible with that motherboard?
No, defiantly not.
In fact, if your computer was a pre-built (Dell, HP, eMachines, etc.), it is doubtful that it will accept any newer CPU.
You would probably be much better off spending $300ish on a new motherboard, CPU and RAM.