I believe this has been reported as a bug in the program, but -as of yet- no details on the fix have been released.
To manually delete a scheduled scan:
To manually remove scheduled scans, do the following:
1. Close Ad-Aware
2. Go to the start menu, choose 'Run...', type in 'Services.msc' and in the service window, stop Lavasoft Ad-Aware 2008 Service
3. Go to the folder C:\Documents and Settings\All Users\Application Data\Lavasoft\Ad-Aware 2008 and delete the file schedule.aaw
4. Start Ad-Aware - the scheduled tasks should now be gone.
Please also contact the support center to inform them that you wish to report this as a bug.
Hope that helps,