These days you simply can't be too careful - especially on the internet. The incidence of cyber crime is one the rise, especially in terms of identity theft crimes. No longer do you just have to be careful about downloading email attachments. Spyware can be loaded and installed onto your computer simply by visiting an infected or malicious website.
Malware can be used for various criminal purposes. These range from general vandalism to stealing information. From a profit point of view, malware is generally used to direct you to unsolicited advertisements or affiliate software (the criminal gets paid a percentage of the sale if you purchase the software), to monitor your web surfing or steal your data / passwords.
For example, keyloggers continue to be a huge threat thanks to the way that they can record all of your keystrokes and then send this information to a third party - presumably the one who sent out the malware in the first place. They can then see everything you've been typing, including usernames, passwords, and credit card numbers. Recently someone I know had all his websites hacked thanks to a keylogger program that had been installed on his computer directly from a website. This cost him quite a bit of time, money, and frustration.
If you do a lot of business on the internet - whether it is personal banking or simple Christmas shopping you should ensure that you have covered all your bases in terms of protecting your PC from malware. The easiest way to do this is to ensure that the program you choose covers an array of different threats.
Some software tools attempt to block malware in real time, meaning that it will try to stop the software from being installed in the first place. Others don't do this but rather try to keep an eye on the websites you're browsing, making search results with red x's if the website is known to be problematic. This is helpful in avoiding phishing sites and other potentially bad apples.
If you don't currently have an an internet security software program installed on your computer I highly recommend checking out the top sellers in the genre. It's an investment in your personal security and identity.