Give this a try:
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.
From Arianae:
If that doesn't work, try logging in with the
Administrator account. I've run into several issues
where even though my account is a member of the
Administrator group, only the Administrator account
itself has done the trick. As a matter of fact, I just
had to fix a disappearing XP clock with it even though
the user was an admin.