It's rare that I will post a whole article, but this excellent article by Neil J. Rubenking is posted today on pcmag.com at http://www.pcmag.com/article2/0,2817,2416788,00.asp
At the same time I refer you to my ow recommended security suite. Read here: http://pcdocsblog.blogspot.com/2013/02/my-pc-security-baseline.html
Here is Neil's article in its entirety:
"Computers are complicated enough that they don't
always do precisely what we expect. Sometimes an unexpected behavior is
just a fluke; other times, it's an outward and visible sign of an inward
and terrible malware infestation. If you notice any of these security
warning signs, your system may well be compromised.
1. Popup ads appear even when no browser is open. While not as
common as they used to be, adware programs bombard their victims with
advertisements. Sometimes they're ads for legitimate products, other
times they contain links to malicious websites, sites that will attempt
to drop more malware on your PC.
2. Browser navigation gets redirected. Not every site redirect
is malicious, but if you find that trying to reach Google takes you to
an unfamiliar search site, you've almost certainly got a problem.
Sometimes the redirection is more subtle. For example, a banking Trojan
might divert your browser to a fraudulent site that looks just like your
bank's real site. In that case your only clue is the unfamiliar URL in
the Address bar.
3. A security program you never installed pops up scary warnings.
Creating and distributing fake antivirus programs is a lucrative
business. The perpetrators use drive-by downloads or other sneaky
techniques to get the fake antivirus onto your system, then display
scary warnings about made-up threats. Naturally you have to register a
payment before the fraudulent tool will "fix" the problem. And of course
scanning for malware with the fake AV is super-fast, since it's not
actually doing anything.
4. Posts you didn't write appear on your social media pages.
Malware focused on Facebook and other social media sites propagates by
generating fake posts. Typically these posts include an inflammatory
statement of some kind, like "OMG were you really that drunk? Look at
this picture!" Anyone who falls for the fake and clicks the link will
become the next victim.
5. A program holds your PC for ransom. Some malware programs literally hold your PC or data for ransom. Overt ransomware
threats may encrypt all your pictures and documents and demand that you
pay to get them back. Others try to obscure what they're doing. For
example, they may display a warning supposedly from the FBI stating that
your computer was used to send spam and demanding that you pay a fine
before you're allowed to use it again. Of course, even if you do pay,
you may not get your system back.
6. Suddenly you can't use common system tools. A smart user,
suspecting the presence of malware, might launch Task Manager to
investigate, or check settings using Registry Editor. If you suddenly
find that trying to use these or other system tools triggers a message
saying your Administrator has disabled them, it may well be an attempt
at self-defense by malware on your system.
7. Everything seems perfectly normal. That's right. Some types
of malware do their best to hide all activity, leaving no visible
traces. Even when you don't notice anything unusual, it's possible that a
'bot on your system may be quietly awaiting instruction from its
command and control system, or a Remote Access Trojan may be harvesting
your personal information.
If you think that malware has taken up residence in your PC, install a powerful antivirus utility or security suite
immediately. Already got one? Then apparently the malware got past its
protection. Make sure your antivirus is fully up to date, and run a full
scan. Also get a second opinion from a free cleanup-only antivirus like
Malwarebytes Anti-Malware 1.70 or Comodo Cleaning Essentials 6.
You definitely want to get that nasty, malicious program out of your
system as soon as possible, before it invites "friends" to make your
security problem even worse."