In W2K And Win NT 4.0 it is located on:
\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion, InstallDate (as a REG_DWORD)

I beliefe that in win 9x you can look in:
\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion, InstallDate

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Lon Geefshuysen (one of the Moderators)
Microsoft Certified Systems Engineer

Senior ICT Engineer
kixtart.to/lon
lon@kixtart.to