Many computer users don't consider the consequences of their machine being virus infected until it's too late! Aside from losing everything, you may also cause your machine to run slow or perhaps even infect others. Here's some tips courtesy of McAfee and other sources along with my comments:
Do not open any files attached to an email from an unknown, suspicious or untrustworthy source. (often times you can infect your machine even from a trusted source!)
Do not open any files attached to an email unless you know what it is, even if it appears to come from a dear friend or someone you know. Some viruses can replicate themselves and spread through email. Better be safe than sorry and confirm that they really sent it.
Do not open any files attached to an email if the subject line is questionable or unexpected. If the need to do so is there always save the file to your hard drive before doing so.
Delete chain emails and junk email. Do not forward or reply to any to them. These types of email are considered spam, which is unsolicited, intrusive mail that clogs up the network. (Many of these chain emails are from "email harvester" looking to collect valid email addresses to use for spamming. Many of these are hoaxes. Check with Snopes.com or another reputable urban legend site before forwarding these emails. You'd be surprised how most of these emails are fake or were real but from years ago!)
Do not download any files from strangers. (be very careful here! Most of these are infected. Often times you may not see any immediate damage, if at all, but your private information, like passcodes or banking info, may be forwarded to a 3rd party)
Exercise caution when downloading files from the Internet. Ensure that the source is a legitimate and reputable one. Verify that an anti-virus program checks the files on the download site. If you're uncertain, don't download the file at all or download the file to a floppy and test it with your own anti-virus software. (watch those file sharing sites)
Update your anti-virus software regularly. Over 500 viruses are discovered each month, so you'll want to be protected. These updates should be at the least the products virus signature files. You may also need to update the product's scanning engine as well. (I would recommend not renewing the definitions at the end of your subscription. It's better to buy the latest version. I was infected by a worm with up-to-date definitions. I called Symantec/Norton, only to discover that my version didn't check for the latest type of worm. The newer software would have stopped the infection.)
Back up your files on a regular basis. If a virus destroys your files, at least you can replace them with your back-up copy. You should store your backup copy in a separate location from your work files, one that is preferably not on your computer. (You can't imagine how bad things would be if you lost your data base! How about losing your email list? Not only can a virus wreak havoc, but what about a harddrive failure!? I own about 15 machines and I've lost about 5 harddrives in the past 3 years! It will happen to you at some point. In addition to backing up my data, I use Norton Ghost to make an image of my harddrive. This way, if my harddrive fails, I can restore my machine without having to reinstall programs, some of which I no longer have or that I downloaded)
When in doubt, always err on the side of caution and do not open, download, or execute any files or email attachments. Not executing is the more important of these caveats. Check with your product vendors for updates which include those for your operating system web browser, and email. One example is the security site section of Microsoft located at http://www.microsoft.com/security. (Updating your system is one of the easiest ways to prevent infection. Many people don't realize that if you turn your computer off at night, you may not be getting the updates downloaded automatically and you may need to check for updates manually.)
I highly recommend you download LavaSoft's Ad-Aware and Spybot Search & Destroy. You can find both by doing a Google search. They are free and can be used together on the same machine to clean yourself of "spyware" which will also slow down your machine.