Okay - sure I know you can BUY software to prevent SPAM but the easiest way for people who don't usually get e-mails from people they don't know is the following:
(Note: this works better in Outlook 2003 because you can specify the Address Book instead of the having to specify each person)
1. Create a rule for messages when they arrive.
2. The rule should be to move ALL messages to a Junk Mail folder but with the following exception:
a) in Outlook 2003
- except when user is in your address book
b) in all other versions
- except when user is (and then you have to select everyone's name)
Doing this - I have been able to cut down my SPAM by about 90%.
Much happier now - and it didn't cost me a penny.