Viruses, spyware, adware (these are commonly called malware, software that is designed to operate maliciously on your computer), are just waiting to get on to your computer.
You should be running an antivirus program. I do not like the two biggies, McAfee, and Symantic (also know as Norton) are quite intrusive, that is they want to take over your machine. Again there are some free alternatives for antivirus. AVG makes a free antivirus for home use, and ClamWin is an open source project for virus protection. E-mail attachments are a common method for sending malware, normally these are what are called trojans. The name trojan goes back to the Trojan Horse, the writers try to entice you to run the trojan thinking it has value for you, but they just want to get into your machine.
Currently there are three main things the malware is trying to do is either display advertisements on your machine. Some directly will try to monitor your machine and send any bank accounts and passwords to someone so they can use this information. Secondly they want to take over your machine to send more malware to other machines (turn your machine into a bot). Finally they want to display advertisements on our machine to entice you to spend your money.
The main symptom of these is you machine does not operate properly, mostly it slows your machine to a crawl.
I have found a good free solution to find malware installed on your machine. DrWebFree Cureit is available at http://www.drwebfree.com/en/cure_it.asp. You down load this program to your machine, then you run the program. By default is scans the memory of your computer where programs are running and the normal installation locations. You can do a complete scan of your computer. DrWebFree cureit is a good way to double check your current antivirus and clean up an infection.