SYMPTOM #1: If your system has slowed down, or if you want to go to one website and wind up at another, or new buttons and new favorites appear that you dont remember adding, or you get constant pop-ups on your desktop, your system is ALMOST CERTAINLY infected with viruses and spyware!
Also, if you used to be able to get on the internet, but cannot now, spyware can often do this!
You can spend hours talking to your ISP only to be told ‘we dont know whats wrong’

What to do?  Most often, folks get BAD advice at this point. 

BAD ADVICE #1) They are told to reformat their drive, or use the system restore disks that came with the computer.
The problem with this (HORRIBLE) advice - is that you lose ALL the information on your computer - the system restore disks usually reset the system to the state it left their factory in.  And when the computer left the factory, it did NOT have your word documents, email address books and the like on it.  This information is (almost always) much more valuable than the computer itself!

BAD ADVICE #2) Some screen pops up on your desktop and says ‘Your computer is (or maybe) infected - click HERE for a free virus/spyware scan’ - or something similar.
THIS ADVICE is coming to you FROM a spyware or virus program - and if you follow it you will be letting spyware come and live with you.
In the words of the Capitol One ad - ‘THE ANSWER IS ALWAYS NO!’

BAD ADVICE #3) GO OUT AND BUY ANTI-VIRUS/ANTI-SPYWARE software!
Problem is that the BEST anti-virus and anti-spyware software - is FREE! - and folks spend perfectly good $$$$ on inferior products.
ALSO - there is an increasing class of spyware nasties out there that can be detected by various of the anti-virus/spyware programs - and which even offer to delete the nasty for you - but simply cannot do so.  It so SO embedded in the XP architecture that any attempt to delete it by ANY automated program is doomed to failure.
I’ve been cleaning these off for years manually - and it involves literally scrolling through directories and locating suspicious files by eye.  NOTHING can automatically take care of this class of critters.

GOOD ADVICE:  Bring the system on down and we’ll put it back in good working order, and we will install the best antivirus/anti-spyware tools out there on your computer to try and protect you from these headaches.