This was a bit of a doozey for me, but very simple to find out.
Go to a command prompt window and type "systeminfo"
This will spit out a whole heap of info. The "Original Install Date" line is what you're after, somewhere near the top.
If you've got a lot of patches installed, it'll flood that line off the screen.
To prevent that, save the output to a file instead.
Type "systeminfo > output.txt". When it's done, open up the file.