First you need to make sure you are running an account with administrator privileges.
Second I would like to know if your computer is in a domain. If so, this setting can be controlled by the domain group policy. Therefore, the buttons for some programs are grayed out. However, you may still be able to control some third-party software at startup. For my system, these buttons for Microsoft Windows Explorer are grayed out but for Messenger and Adobe Acrobat are not grayed out.
Actually, we can enable or disable startup programs using the following method:
1. Click Start, type "MSCONFIG" (without the quotations) in the Search Bar and Press "Enter" to start the System Configuration Utility.
Note: Please click Continue if the "User Account Control" window pops up.
2. Click the "Services" tab, check the "Hide All Microsoft Services" box. Uncheck the box before the items which you would like to disable at startup.
3. Click the "Startup" tab, click "Disable All" and click "OK". Uncheck the box before the items which you would like to disable at startup.
4. Restart the computer and these services and programs will be disabled at startup.
Please understand that we mainly discuss Deployment issues in this forum. As this issue is mainly related to Windows Defender, I recommend discussing it in our Windows Vista Applications forum.