I've been searching for a solution to this problem and finally figured it out so here it is for others still in the dark:
1. open up regedit (start --> run --> regedit)
2. navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Products
3. Search for "Microsoft Security Client" which should be a subbranch of the above directory
- it will look like "some random string"\SourceList\ which contains Media and Net
4. Right Click on that random string and select Export to back it up in case something breaks
5. Right Click on that random string and select Delete
6. Rerun the installer
This worked for me, and it will also work for any program that gives an error like this, not just Microsoft Security Essentials
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