There is a particularly nasty bug that tries to trick you into installing it by flashing a notice that "YOU ARE INFECTED" and telling you you must download the recommended software immediately to fix the problem. And when you innocently do that, you have now given the virus permission to install itself on the machine. This virus or Trojan is very difficult to remove and has occasionally been impossible to remove, rendering the machine useless.
There are also a number of pop-ups that warn you that you have spyware and telling you to click on their button to have it cleaned. And when you click, it actually INSTALLS more spyware on your system. The more benign versions try to trick you into buying the “paid” version of their software.
No antivirus program can defend against a virus we have given permission to invade our system.
If you make sure you know what security programs you have installed on your computer (or on company computers), you will not fall prey to these traps. If you know you have, for instance, Norton Antivirus and Ad-Aware antispyware, and another program offers to help you – you know it’s a fake! You also need to be aware of the wording – “Ad-Aware” is an antispyware program, “Adware” is NOT. Microsoft will NOT warn you that your IP address is compromised, and Windows will NOT discover spyware on your system. Oh, and they will NEVER call you and tell you that you are infected, either. Never.
Best practice is to stick with what we know –
For antivirus software, install Microsoft Security Essentials (it's free at www.microsoft.com/en-us/security_essentials/ ) or NOD32 (www.eset.com).
For a another free decent antivirus product, try AVG Free (at free.grisoft.com).
Install only ONE antivirus product – they don’t work better in multiples, they just conflict with each other.
And just don’t click on those warnings…they are meant to scare you (and they do it well!) into infecting your own system.