Around 75% of the mails that we receive on your email is said to be spam, says a survey. Yes, spams are the most common and most annoying threat to your PC in an indirect way, as it would clutter up your inbox of your ISP and hence might lead to bouncing of important mails that you might receive. Some spams too will be a security threat for your PC, so never every open up a mail that you feel is from an unknown source and even if you have opened it, never download anything or click on the links. Install an antispam software in your PC.
While browsing the net there may be a common irritant which comes pop when you visit certain sites called pop-ups. These are ads which are served to you to check out when you visit some sites. These pop-ups could be very annoying when you are researching the net for some important info. To avoid such pop-ups you could install pop-up blockers to your browser. The most common browser internet explorer does not come with and hence you can install the pop-up blockers through third-party toolbars such as Google Toolbars or Yahoo Toolbars, etc.
Some browsers come preinstalled with pop-up blockers and other security features bundled along with it. Firefox which is an open source software is one of the best known browser which is more secure to browse on the net than most other browsers. Hence you can install Firefox and start using and be secure.