Scheduled scan is a known buggy feature in MSAS Beta 1, give this a try
first:
Open up control panel and double-click on add/remove programs.
Select Microsoft AntiSpyware
Select "Change"
On the Microsoft AntiSpyware Maintenance Wizard, click next.
On the next screen (Microsoft AntiSpyware Maintenance Wizard) select Update
Microsoft AntiSpyware and click next.
Select Install
Let the product update.
Click Finish.
If that does not work, try uninstalling/reinstall.