Make sure your computer has an anti-virus program in use. Keep backup copies of important files. Accidental deletions or similar mistakes are much more common than virus attacks.
Backups are a cost-effective way to deal with both problems.
Downloading untested software from the internet is another potential source of viruses. Be cautious of e-mail attachments from unknown sources. Occasionally scan for viruses in all files. ESafe's ViruSafe Web is a free browser plugin that scans files you download from the web for viruses.