This page needs a complete re-write with the "End of Life" of windowsXP. 
But here's what to do if you have a XP machine and if the
'no longer supported' XP makes you feel uneasy:  MINT

Below is the pre-EOLXP content  RETURN now or read on.

If you are thinking of just not renewing the subscription because you haven't been bothered by virus's, try this: Open your present antivirus program and search for a log viewer, or reports, and see how active it's been in silently deleting or denying access to suspect files.

You want to be in control of your computer, right?

So don't let a virus take over.

Here's an article that will point out The 17 Most Dangerous Places on the Web

Not only is your computer vulnerable, your smartphone is to.
Cybercriminals are making their attacks personal, remotely locking your computers and smartphones until you pay a hefty ransom. C/Net

Here's the report I made for a computer that got hit with some "scareware" that made the computer totally unusable. Why did this happen? The antivirus, pre-installed and included in the purchase, was only a 90 day trial. It was not purchased, subscribed, in the 90 day "window". It was not removed and another antivirus installed.
Result after 90 days, NO PROTECTION. REPORT

There's a host of very good anti-virus programs available. Don't operate a computer on the internet without one installed and up to date.
Here's a list of Consumer security software providers (Anti-virus)

If you can't afford one of the commercial versions, or just think this is something that
should have been dealt with at the Operating System level -
Microsoft does now have FREE* antivirus.
*Free for personal/Home or up to 10 computers in a small business. NAG FREE.

They call it Security Essentials. and is available here: http://www.microsoft.com/security_essentials/

UPDATE: XP End Of Life and Security Essentials

If you' feel better with a 3rd party anti-virus solution, Here's a 2012 comparison

Other things not classified as a virus but may be some program used by advertisers tracking your internet activity or serve advertisements directly to your computer screen, Some will actually hijack the pages you view, replacing the real content with their own.
(Most of these "hijackers" are dealt with by antivirus software.)
Most cloak their actual function as a useful toolbar or helper object in your Internet browser.

Then you can "back up your anti-virus" with a scan occasionally with Malwarebytes Anti-Malware
It's not anti-virus, but will clean out the advertiser tracking crap, and such,
that anti-virus often overlooks that can slow your computer and cause other problems.
Malwarebytes Anti-Malware download at:
http://download.cnet.com/Malwarebytes-Anti-Malware/3000-8022_4-10804572.html
It comes with 30 days of "Real Time" protection which can be extended with a paid subscription (not really necessary, and no NAG )
The scanner is free to use without a subscription. It's recommended all over the internet by Computer tech guru's. You can run it now and then, at you leisure.

Have a good day, and many to follow

RETURN

You've already lost control to some virus or malware?

if you can describe your symptoms and
have a general understanding on how your computer works, and
If you are capable of following directions. and
if you are comfortable with getting into the "heart" of your computer,
You may get one on one help from one of the following  forums:
SpywareInfo, Bleeping Computer, Geeks to Go, Gladiator Security, Malware Removal, WhattheTech (formerly TomCoyote), Smokey-Services 

Maybe this could be your first step:
http://microsoft-standalone-system-sweeper.en.softonic.com/