All packages registered with Windows Installer have an entry in this location.
Please note that this location can also be found under "Wow6432Node" on 64 bit machines. The following article
from MSDN can give you more details.
Approach you can try if you know the old version product code:
Simply add a registry search from the Search page using the old version Product Code using one of the appropriate registry values( above article ) that holds the old version:
Approach you can try if you don't know the old version product code:
You can use the information from the registry key in your own custom action to get the installed old version. In the same custom action you can get a list with the Product Codes of the older versions of your installation package from the OLDPRODUCTS property. This property is set only in a package which upgrades another package. The custom action should also be conditioned using OLDPRODUCTS.
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All the best,