what is the diffrent between celeron, pentium 4 , Xeon and pentium D ?  

I believe it is the cache size...

I recommend Pentium 4...

A Celeron is your basic processor. It has little onboard cache memory, and is generally less powerful than the others.

It will do fine for basic tasks that don't require much power.

A P4 has more onboard cache and is quite good, although it has only one core. It may have hyperthreading and it may not depending on the type and design.

It is suitable for more powerful jobs and is quite good.

A Xeon is for the server market, is expensive and not generally used for home computers.

A Pentium D is the Dual Core answer to the P4. It has the same power characteristics in general.

A dual core is like having 2 processors in one package. It is able to do the work of two, and will allow many applications to be used that used to bog down a single core processor.

Now the Core 2 Duo is out, and soon there will be a quad core.

These newer processors feature more power and require less power to run than the previous models. They have better heat dissipation and are built on a smaller architecture than previous processors.

I'm not much familiar with them. All I know is that I know how to operate a computer. Maybe you would like to share it with me.