I have both AdAware and SpyBot. I run them once a week to make sure the "junk" is off my machine. Since I started using them my spam has dropped about 30% - 40%.

Another thing you can do to reduce your spam is to get a "throw away" email address (Hotmail, Bigfoot, etc.) Use the throw away address for when it asks for an email address. Only use your correct address IF YOU ABSOLUTELY HAVE TO (if you order something, your friends, etc.) NEVER RESPOND TO SPAM AND NEVER USE THE "CANCEL THE EMAIL/FUTURE SENDINGS", IT CONFIRMS YOUR EMAIL AS VALID AND THE ADDRESS WILL BE SOLD TO OTHERS.