Winantivirus is one variant of several "rogue" security products.
This rogue can be picked up in several ways. Casual surfing, by simply clicking to download it yourself, or a "drive-by" download. The site you are on requests the download of a file needed to make the site work. You will be unaware of it.
Or when you visit a site and a popup appears that mimics the appearance of your real antivirus product, telling you that you have a virus, and by clicking the warning you can remove it. What in fact happens, is that clicking will instigate the download of the real trojan.
If you use file sharing programs like Edonkey or Bearshare, etc to grab music and files, then be aware that some of these files will not be what they say they are. Likewise, if you download files that are apparently free "full versions" from certain forums and message boards, there again, you may get more than you bargained for!
One other avenue of entry is by exploits that exist in your system software. Never go online without all the available patches!
Once the rogue has hold, what will follow is false scans and many popups informing you that you have become infected with an overload of virus, trojan, and other malware items. In order to clean the system you need to purchase the full product. Save for this rogue itself and whatever trojan was used to install it, the list of infections is a hoax. It is an attempt to frighten you into parting with money that will bring no benefit whatsoever. Just think what will happen to your credit card details you just paid with......
The very first thing to do is become more aquanted with your security products. That doesn't mean to say you should try to gain in depth knowledge about them, but learn what they look like, and their characteristics. Go through the various windows the application offers. Look at the help file, which usually gives shots of what to expect when something is detected. This is a must, since Winantivirus and similar malware of it's type look very convincing to the unwary. A quick reaction click is all that's needed to con you.
Supposing you see a popup window telling you that a virus has been detected. The immediate response may be to panic and hit the remove button. Don't! Instead, examine the look of the window to ascertain that it is from your own security product you know to be legitimate. It would be appropriate for the vendor name to be shown properly. That being the case, it would be safe to continue. Anything other than that is cause for concern. Keep a cool head and click nothing.
Normal scans via your installed Anti-virus/malware product would be appropriate, though this isn't failsafe. Actual removal instructions are beyond the scope of this article, so I will send you in the direction of Bleeping computer. These people are well versed in the art of removing such things and will help you greatly.
The internet is always going to be a place where people will be preyed upon, so do all you can to maximize your chances. Keep system and security products fully updated. Learn the basics of self protection. It needn't cost you anything at all. It could save you a lot of money and heartache.