Windows 7 - Dual core 2.6 and I3 core 2.2 or I5 core 2.2?

whats the differance between pentuim dual
and wich is the faster ?

No contest, any i3 or i5 will pretty much eat a Pentium D's breakfast, lunch and dinner regardless of clock speed.

Does I5 have only 2 cores ?
and i7 have 4 cores ?
So pentuim dual core 2.6 isnt faster than i3 core 2.4 ?

It depends on what i5 and 17 you are talking about. What are the numbers behind the i5 and i7, and are you talking about the new versions such as the i5-2500K or are you talking about the first gen, i5's? The first generation i5's have 2 cores up to the i5-750 which has 4 cores. The number of cores in the i7 depend on which i7 you are asking about. The best way to answer your wuestions is for you to look at the processors. Here are the 1366 CPU's and here are the 1156 CPU's. Look the ones up you are looking for and the details will answer your questions. Here are the newer 1155 Cpu's.

Is pentuim dual core 2.6 faster than i3 core (sandy bridge ) 2.2 ?

CPUs are not always ranked up by their speeds. the Sandy Bridge CPUs are much newer and have Hyperthreading

The Pentium Dual Core CPUs are obsolete and weaker than the modern CPUs

ok I got that ,I want to be sure about another thing ,when I find I5 core 2.0 quad cores does it mean its 8 Ghz?

No. There is no 8Ghz CPU. That is just about the world record Overclock.

isnt each core 2.0GHZ and its 4 cores then the CPU is 8 ?!