Due to the porous nature of Windows all sorts of malware (virii, adware,keystroke loggers, spam senders) can slide right into your computer from the Internet, from file sharing, from thumb drives, and just plain out of the air. There must be a malware magnet deep inside Windows. If you want to call your computer your own, you have to do something the keep the bad stuff out.
New machines come with something, usually Norton Anti Virus or McAffee Antivirus pre installed and pretending to be free. After some months, the free wears off and the programs beg for money to "keep them selves up-to-date". I used to run Norton Antivirus, but when it begged for money, I took a hit to my credit card but the program still refused to update and shortly just stopped running. Unless kept up to date the program quickly becomes useless. Scumbags from all over the world are constantly writing new malware, and unless updated, the anti virus program won't recognize the new malware when it strikes.
After the Norton failure to update, I started using F-Prot, an old DOS antivirus. So, starting up son's new laptop, I put F-Prot on it and ran it. Surprise, F-Prot only scanned 300 files and declared the computer virus free. That can't be right, even a virgin Windows computer has tens of thousands of files on it. Some googling on F-Prot and "long file names" revealed that trusty old DOS programs , raised on short filenames (8 character name plus three character extension), have trouble with Windows file name that can be of any length and even contain embedded spaces. So much for trusty old DOS antivirus.
Some more googling put me intouch with AVG antivirus. It's free, it updates itself, and it is a real Windows program in which the mouse works. It's 45 Megabytes to download, but broadband makes that happen with ease and grace. It is painstaking, examining file after file.
So if you want to save the maintainance fees of the commercial antivirus programs, try AVG.