What is the basic difference between a present day processor(Pentium IV ) and a Digital Signal Processor ?

Answer 1

A Digital Signal Processor (DSP) has a very specialized hardware which is ideally suited to making digital filters. It has one or more multiply/accumulate registers as well as memory pointers which work in a circular fashion so that they can efficiently cycle through coefficients. Many functions can be working in parallel.

The same functions can be performed in a Pentium however the Pentium is much more general purpose and may require several instructions for something that can be done with one instruction in a DSP.

Obviously there is a blurring of the differences and the usage would depend on your application.

Also in general DSPs would be used in embedded type applications, not a general purpose PC.