TL;DR version: No. Don't touch unless you know what you're doing.
As for details, I will take Triumph's approach here and hit you with research:
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Quote:
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MSI
MSI is an installer package file format used by Windows. Its name comes from the program's original title, Microsoft Installer, which has since changed to Windows Installer. MSI files are used for installation, storage, and removal of programs.
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Source
In this case, the emphasis is on storage and removal. That folder contains files necessary for program updates and clean uninstalls. If you happen to delete something, a program may not work properly. Especially if it's a program you don't use very often, since you'll only notice the results some time later.
Here's another post which goes into a little more detail about %windir%\Installer directory:
Details
EDIT:
Have you tried using
Windows Disk CleanUp Tool?
Threaded Mode
