Do you have any other security software installed, such as SpyBot Search & Destroy? These can identify attempts by one program to integrate with another as being evidence of malware settling in, and block the changes.
If Norton is the only security software you have installed, have you recently upgraded it, ur installed it to replace a different company's security product? There can be problems like this that could arise during those processes if you don't follow a very specific set of procedures.
Tell us a little more about how you come to be in this situation. Have you ever had the Norton toolbar present in Outlook, or is this a new install (either of Norton or of Outlook)? If the toolbar worked before, what happened--either things you did or things you noticed--shortly before it went away?
The more we know about your situation, the more we can do to help.
Can you check if Norton AntiSpam Plugin has been disabled in Outlook 2007 settings?
To do this, please go to Outlook Menu - Tools>>Trust Center>> Add-ins(in the Left Pane)>>Go to 'Manage COM Add-ins'
Check if Norton AntiSpam Outlook Plugin is listed here and is checked. If unchecked, please check this and this should resolve your problem. Let us know what you see..