It may surprise, but you are not alone in receipt of e-mail from people you've never heard of. Estimates are that 80% of current e-mail across the 'net is spam and virus e-mail.
First things first; replying to spam is a recipe for more junk. Unscrupulous operators use your reply as confirmation of a valid address. Secondly, this is not strictly true for Australian originated spam, because it is now illegal - you can find detailed information and advice at: The Australian Communications and Media Authority
What else can you do?
You can filter much of the junk, but you run a (slight) risk of filtering a legitimate message because none of the software currently available is one hundred percent accurate - most doesn't come close to 80%.
Protect your address like a credit card number. Use caution when using your address on web- sites or sending jokes to your friends, who in turn may forward it on. At some stage all those addresses end up in a database ready for use.