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Exchange 2007 has a built in anti-spam filtering module that just needs to be enabled.

Here are some simple instructions to get going with it. Once you’ve enabled this feature, you should see an anti-spam tab in the Exchange management console under Organization configuration > Hub Transport > Anti-Spam.

To enable anti-spam functionality on a Hub Transport server

1. Open the Exchange Management Shell

2. cd to the scripts folder in the Exchange install directory.
%system drive%/Program Files\Microsoft\Exchange Server\Scripts folder.

3. Run the following command:

./install-AntispamAgents.ps1

4. After the script has run, restart the Microsoft Exchange Transport service by running the following command:

Restart-Service MSExchangeTransport

5. Enable automatic anti-spam updates.

Anti-Spam Updates

Forefront Anti-spam Automatic Updates is a premium feature that requires either an Exchange Enterprise client access license (CAL) for each user mailbox, or a Forefront Security for Exchange Server license.

You can see how to configure automatic updates here.

Some info for this article was taken from these Microsoft articles.

http://technet.microsoft.com/en-us/library/bb201691(EXCHG.80).aspx
http://technet.microsoft.com/en-us/library/bb691338(printer).aspx