Knowledge and the ability to use it is the best defensive tool anyone could have.
This includes educating yourself as to the from Microsoft which will patch many of the security holes through which attackers can gain access to your computer.
Even then, in some cases, this opting out does not always seem to work as intended.
Beware of rather then standard anti-virus definition (signature) detection software only.
These kits come with pre-written exploit code and target users running insecure or outdated software applications on their computers.
and change them anytime you encounter a malware infection, especially if the computer was used for online banking, paying bills, has credit card information or other sensitive data on it.
Every security vendor's lab uses different scanning engines and different detection methods.
Each has its own strengths and weaknesses and they often use a mix of technologies to detect and remove malware.
This would include any used for taxes, email, e Bay, paypal and other online activities.
You should and change all passwords immediately as a precaution in case an attacker was able to steal your information when the computer was infected.
The security community is in a constant state of change as new infections appear and it takes time for them to be reported, samples collected, analyzed, and tested by anti-virus vendors before they can add a new threat to database definitions.