Try manually restarting the service by following these steps:
1. go to the start menu, click 'Run...' and type 'services.msc'
2. in the services window, locate 'Ad-Aware 2007 Service', select it and click 'Stop'
3. when the service is stopped, click 'Start'
4. When the service is running, try starting Ad-Aware 2007 again.
If hat doesn't work and you are using Vista come back.