If you are planning to install Microsoft Security Essentials in your PC, you might face some issues during installation, in that case you could follow this checklist to resolve them.

Case 1: Already Infected PC: If your PC is already infected with Malware, then you have to clean it before installing Microsoft Security Essentials. There are Malwares that block Anti-Virus installation or interrupt during installation. The best way is to do a deep Malware clean up, which is using Windows Defender Offline . Find a PC that I safe (no Malware infection or security issue) and download and create a bootable scanner using DVD/CD or USB. Make sure to check your system and ensure that your BIOS could boot USB ( some old BIOS doesn't support boot from USB and in that case you should use DVD/CD). And then run a full system scan with Windows Defender Offline and remove all threats. It is good idea to write down name of Malwares , because you might need to read more about them and see what really happened in your PC. In case that you don't have any other PC and you couldn't use Windows Defender Offline, use Microsoft Safety Scanner and run a full system scan with it. You could try run full system scan in safe mode. Make sure that your PC is not infected with Malware before install Microsoft Security Essentials.

Case 2: Windows Genuine Problem: If your Windows is NOT Genuine or Windows Genuine Validation is failed, you couldn't install Microsoft Security Essentials. Before install Microsoft Security Essentials make sure your Windows is Activated . For issue with Windows Genuine , visit Windows Genuine website.

Case 3: Another Anti-Virus is installed: If you are using another Anti-Virus Product such as Norton, Kaspersky, CA, F-Secure, Trend Micro, etc. You should uninstall it first and make sure it fully removed from your PC and then install Microsoft Security Essentials. In some cases, uninstall it from Programs and Features or Add/Remove Programs won't remove Anti-Virus completely and you need to do a advance uninstallation. Take a look at Advance Uninstall of Security Software and find your Anti-Virus software and follow instruction to uninstall it completely and then restart your PC and install Microsoft Security Essentials.

Case 4: Error message during installation: Run Windows Update and install all updates and restart your PC. If any other update running let it run and complete. If any other program is running or updating close it. In case that issue persist, follow steps on I can't install Microsoft Security Essentials.

Case 5: Windows 8: You could NOT and do NOT need to install Microsoft Security Essentials in Windows 8. Microsoft Security Essentials in NOT compatible with Windows 8 and instead, you have Windows Defender that is Anti-Malware solution and is replacement of Microsoft Security Essentials in Windows 8 and it is part of Windows. When you installed or start your Windows 8 , just type Windows Defender and you would see it.