Just don't give out your email address to people you don't know/trust.
It seems no matter what, we all get junk email.
One thing that can help a little...if you look at a lot of junk mail, there will often be a way to tell them to stop sending you email. DON'T tell them to stop. This only proves they hit an active email address.
If you have MS Outlook or some other email programs, you can designate email as "junk"...and block the sender...you will still get it, but it goes directly into the "junk mail" folder. I don't think Outlook Express (comes with Windows) has this feature, but I haven't used it in a long time.
Using web based email helps, but is inconvenient for some (like me).
NEVER EVER EVER open an attachent from a source you don't know. For that matter don't even open attachments from people that forward you every joke or whatever they receive. You know you are not likely to miss anything, and "trojan horse" viruses can come from people you know....some they sent not knowing, some send themselves...they read your friend's address book and send to everyone in it.
There are no jokes worth opening an attachment for.. There are no porn pictures, movies, etc....The only attachments I will open are if the email itself is from someone I know and they say specifically what the attachment is.
open an attachemnt that has an "exe" extention. NEVER!!!!
Keep your anti-virus software up to date. AVG is very effective and very good and FREE...I have a friend that recommended it....he claims it is better than Norton and he should know (though I use both Norton and AVG just to be more safe)..this guy does computer work for the NSA.
Link to the AVG anti virus site: http://www.grisoft.com/doc/1
Also for spyware (popups, malicious sites, etc.) Everyone should be using Ad-aware, also free and very effective. Check out the site and the awards it was given by different PC publications. "Spybot" is also very good.