There is no 'foolproof' way of stopping junk/spam email I'm afraid, all you can do is take precautions.
To start with, what anti-virus are you running? Personally I use Norton Internet Suite (not just the anti-virus) which has a built in spam filter. Ok it wont stop all junk email, but my tweaking the filter and reporting email as spam you will help stop more coming in.
Also, if you run a website and you have your email address totally visible (i.e click on it and it takes you straight to your email client) then any robot/computer is able to do the same. To safeguard against this, dont make email links clickable, make people have to type the address in manually. Yes I know it's much easier to just click a link and by disabling this option may be offputting to some people, but doing this really does help against automated junk mail.