if i reinstall Mac OS does that delete all my programs i have downloaded

[genkholi]

If i reinstall Mac OS does that delete all my programs i have downloaded? I heard that reinstalling Mac OS will be the equivalent to reformatting a PC. But if so, do i need to reinstall all my programs i have downloaded before? Can i back up all the programs including microsoft office? My macbook is starting to run very slow and i want to "clean it up" to erase some unnecessary stuff and programs i dont need.
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[stream-recorder.com]

Do you have both Windows and Mac OS? How is your Windows installed?