I can't install Microsoft Security Essentials

You might encounter some issues when installing Microsoft Security Essentials. This is usually because you have some other security program running or because you're missing some prerequisites. To help with these issues, follow the instructions that follow. You’ll need to restart your computer, so please print this page or bookmark it.

Note

If you can’t install Microsoft Security Essentials because you can’t even download it, the first thing to try is your Internet connection (but if you’re viewing this page, that’s probably fine). You may also want to try downloading from Windows Internet Explorer.

Step 1: Remove antivirus or antispyware programs

1. Completely uninstall any existing Internet security programs.
2. Restart your computer.
3. Install Microsoft Security Essentials again. If this doesn't resolve the issue, continue to the next step.

Step 2: Ensure that the Windows Installer service is running

1. In Windows 7, click Start. In the Search programs and files box, type services.msc, and then press Enter.
   
   In Windows Vista, click Start. In the Start Search box, type services.msc, and then press Enter.
   
   In Windows XP, click Start, click Run, type services.msc, and then press Enter.
2. Right-click Windows Installer, and then click Start. If Start is unavailable and the Stop and Restart options are available, this tells you that the service is already started.
3. On the Services page, on the File menu, click Exit.
4. In Windows 7, click Start. In the Search programs and files box, type command prompt. Right-click Command Prompt, and then click Run as administrator.
   
   In Windows Vista, click Start. In the Start Search box, type command prompt. Right-click Command Prompt, and then click Run as administrator.
   
   In Windows XP, click Start, click Run, type cmd, and then press Enter.
5. Type MSIEXEC /REGSERVER, and then press Enter.
   
   Note

   There is no indication that this command has succeeded or failed.

   
   
6. Install Microsoft Security Essentials again. If this doesn't resolve the issue, continue to the next step.

Step 3: Make sure you're installing the right version for your operating system

1. Check which version of Windows you're running. For more info, see Which version of the Windows operating system am I running? 
2. Make sure you know whether your PC is running the 32-bit or 64-bit version of Windows. To find out, see Is my PC running the 32-bit or 64-bit version of Windows? 
3. Make sure you download the version of Microsoft Security Essentials that goes with your operating system.
   
   Note

   Windows 8 currently isn't supported. Please see the system requirements page for a complete list of operating systems that Microsoft Security Essentials supports.

Step 4: Start Windows in Selective Startup mode

1. In Windows 7, click Start. In the Search programs and files box, type msconfig, and then press Enter.
   
   In Windows Vista, click Start. In the Start Search box, type msconfig, and then press Enter.
   
   In Windows XP, click Start, click Run, type msconfig, and then press Enter.
2. On the General tab, click Selective Startup, and then clear the Load Startup Items check box.
3. On the Services tab, select the Hide All Microsoft Services check box, and then clear all the check boxes for the services that remain in the list.
4. Click OK, and then click Restart to restart the computer.
5. Try to install Microsoft Security Essentials again.

Step 5: Contact customer support

Error messages for your reference