Disk Defragmenter, or Defrag as it is more common referred to, is a component part of Windows operating systems.  It's basic function is a housekeeping role for all the files on the hard drive.

Spam is the annoying mail you get from companies trying to promote their products, see also phishing.  Usually this type of email is received because you have given your email address to a company either in a shop/office or online.  If the offers you are getting are no longer required then simply unsubscribe to the mail by the automated process by writting to the company.  If you still have a problem you can install some anti-spam software which will filter out the annoying mail.

A firewall is a piece of software that limits the information going into and out of your PC to the internet.  Windows XP and Vista both have a firewall built in which works in the background and is considered sufficient in most circumstances.  It is possible however to buy and install additional software as required.

In simple terms think of your PC as a walled city and the firewall as a guard on the gate monitoring access.

Along with all the spam that you receive you may also have noticed that some emails appear to be from banks, or online shops/auction sites. The sole purpose of these emails is to fraudulently obtain your username, password and other personal information in order to carry out identity theft. This practice is commonly referred to as "Phishing"