Malware / Viruses
At one time most viruses were spread through sharing of floppy disks. Then e-mail became the primary means for delivery. Now with so much internet usage, that is now the number 1 method of getting infected. In very rough terms e-mail would account for 10-20% of the infections, while the web sends out 80-90% of the bad stuff.
Virus Scanning Software
Everyone should have up to date anti-virus software on their PC. Just having the trial Anti-Virus software that came with the PC is not good enough. New viruses are being created every day. Anti-virus software providers generally have updates daily and in some cases hourly to detect the most recent viruses. If you should use the Automatic Download feature of your Anti-Virus software.
Beware of Sender
Even with up to date anti-virus software, you should beware of any e-mail sent to you with an attachment that you do know know who the sender is. There is a delay between the time that a new virus is discovered and the time that the Anti-Virus software is updated so try not to be the one that "discovers" the virus.
If you get a virus
If you do get a virus (or you suspect you have one), do a full system scan with your Anti-Virus software. If it finds anything, have it first try to Clean the file, but if it cannot you need to quarantine or delete the file. If it is a system or critical file you will have to restore it from a clean backup. If a Virus is found you should also go to your software suppliers web page and look up the virus it found and then look at the removal method. Then look at your system and verify that the virus is eliminated, or do the steps to remove the virus.
I meant to get Anti-Virus...
If you are running without Anti-Virus software or you virus file is too far out of date and you get infected, you can still run some free scanners to check for viruses. Emsisoft has a 30 day trial that you can use to scan. Malwarebytes also has a free version that is run on demand. Both Emsisoft and Malwarebytes do a very good job of catching the most recent "Malware".