I know it's retro. But it'll bite you in the butt one day. In particular watch for files with .VBS or .VBE at the end of the file name. In short, go into Windows Explorer, click View, Folder Options, and click the View tab. Don't open or run files attached to messages, unless you know for a fact that the file is clean.
Some common sense helps.
Thankfully some anti-virus software makes checking email attachments a bit easier. Products like Norton Anti-Virus have a feature that will monitor your computer all the time. Whenever you save a file it will scan it for viruses and warn you if it looks unsafe. There's no such thing as a 'trusted source'. Even if your sainted maiden aunt who only uses her computer on Sundays sends you an email attachment - check it.