What is a virus and what software should we have?
A virus is a catch all term used for malicious software. It is an umbrella term used for worms, trojans, spyware, rootkits and many more. Each of these sub-terms mean different things about how the malicious software acts and behaves. It can also describe what its purpose is. For example, spyware is designed to watch your machine and gain information from it and worms are designed to spread on their own with little or no command as it auto replicates. Worms have been know to be highly effective when used in emails as as soon as someone opened the attachment containing the worm it copied its self onto the machine and then sent emails on its self to the whole contact book to spread.
What is needed is an antivirus software. That is the next line of defence. This should keep an eye on everything that comes and goes through the computer to make sure it has no signs of being a malicious piece of software. Most "antivirus" these days pick up more than just "virus", not as well as specialised tools as mentioned later, but they try.
As a note, Avast has been known to include extra software and wants you to install things like a Google toolbar, so be careful what you click.
Even if you are downloading from a respectable website and trusted source, you must be careful as there has been lots of attention brought to Download.com as some of the downloads have been packaged with extra software with their own installers.
If you are unsure of a download, a good thing to go is use a service such as VirusTotal. By going to them you can point them at a link, download or file and have them scan with multiple virus engines to see if there is anything in it.
It is not a good idea to have more than one antivirus installed at one time because they can conflict with each other, but you can have antivirus installed along side antimalware and antispyware.
If you need a second opinion from your antivirus if you think it has given a false positive or has missed something then you can use a service such as Trend Micro Housecall.
Housecall is an antivirus that is an on-the-fly type, you download the file, check for updates to make sure it is current and then just scan.
Trend Micro Housecall
This is not always enough and specialised software is needed that target different types of virus. Even to that extent it has been know to have companies develop special programs to deal with just one virus as a specific tool.