What is a PC made up of ?

Read the An Introduction to PC for description.

What's the most up-to-date CPU for the PC ?

Have a look in the CPU & Chipset guide

Where can I find a good review of current PC Hardware ?

Tom's Hardware Guide has many reviews and detail testing reports.

What's the difference between PC100 & PC133 memory?

This guide will illustrate the basics of SDRAM performance settings, such as system BIOS memory options and memory frequencies, as well as some tips on how to determine the optimum settings for your PC.