Are you absolutely sure it's stored somewhere in the system? the system may keep track of it internally and not publish it to outside eyes. I haven't looked at it but it's just something to consider.
Turns out (according to this site) that you can't get ahold of it easily... but using Win32::API you can "roll-your-own" as they put it here.
Grygonos
Comment on Re: Windows 98: track the current user's idle time