There are many ways to determine when Windows was installed. Here are some of them.
Systeminfo
The systeminfo tool displays a lot of interesting information about the computer and the operating system, among them the installation date. Here is some sample output:
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Host Name: WIN7
OS Name: Microsoft Windows 7 Enterprise
OS Version: 6.1.7600 N/A Build 7600
OS Manufacturer: Microsoft Corporation
OS Configuration: Standalone Workstation
OS Build Type: Multiprocessor Free
Original Install Date: 9/17/2009,