A very common complaint after install Office 2007 goes something like this:

"I am currently using Outlook 2002 as part of Office XP. Installed Office 2007 Standard which includes Outlook 2007, but it won't install. Setup says Outlook as "not available". What's wrong?"

You can only have one version of Outlook installed. You'll need to uninstall the old version of Outlook, rerun setup and click the "not available" icon and change it to "Run from My Computer".