We've recently been asked to rebuild and take a look at the most common type of computer problem, a virus attack. These are very common, and only really combated by the use of a Virus Protection program.
If you have these programs, then make sure they are updated reguarly and set to automatically update the definition files.
Viruses are made and dispatched every day, and the updated definition is needed to combat any attacks that may happen.
One of the main sources of recieving a virus is from emails. Make sure that your email programme is set to scan for viruses automatically when both sending and recieving mail. Also always be weary of those emails that arrive with no subject header and an attachment.
Another thing to look out for are emails that are forwarded to you, saying that you need to delete a file from your computer. Never do so before consulting a computer "expert" first. These are usually hoaxes, and can cause a very important system file to be deleted and may even crash your system.