Spyware is a category of malicious software that secretly obtains information about a computer user's and sends it to a third party without the user's consent. For example, a piece of spyware running on an infected computer can obtain the user's bank account number when online banking, user's username and password when online trading or the user's credit card number when shopping online.
No computer user is 100% safe from spyware, however it is primarily a concern for users of Microsoft Windows operating systems. The safest bet to avoid being a victim of spyware is to use an alternative operating system like Linux or Mac OS X, however if you are unable or unwilling to leave Windows, there are some steps you can take to protect yourself.
Internet Explorer contains a technology called ActiveX that allows malicious web sites to install software on the browser's computer without the user's consent, this ability has been a boon for spyware writers and distributors. Alternative browsers include Firefox and Opera.
Outlook express contains several security vulnerabilities spyware distributors use to infect computers. Instead of Outlook express you can use an online email system like GMail or Yahoo mail, or install an alternate email client like Thunderbird.
Hardware firewalls can be obtained for a reasonable price these days, most routers (both wired and wireless) provide firewalls in addition to their primary purpose of allowing multiple computers to share broadband internet connection. Software firewalls can be installed on Windows computers as well.
While nobody is completely safe from spyware, by taking these simple measures the risk of getting infected are greatly reduced.