Hidden Periodic Screenshots on a corporate workstation?

You can use nircmd. Of interest is this part:

Save 10 screenshots in a loop, and wait 60 seconds between the screenshot save calls. The filenames of the screenshot will contain the time and date of the saved screenshot.

nircmd.exe loop 10 60000 savescreenshot c:\temp\scr~$currdate.MM_dd_yyyy$-~$currtime.HH_mm_ss$.png

You can start it remotely with psexec like so:

psexec.exe -u user -p pass -i (remote session id) \\hostname c:\remotedir\nircmd.exe etc.

In my opinion (and experience), technology rarely solves behavioral problems. If your problem user is essentially not working when he or she should be, I suggest the appropriate supervisor take him aside and in a non-confrontational manner bring up the issue; something along the lines, "Look we think you're not pulling your weight here and that you are abusing your computer privileges... this behavior isn't what we expect out of our employees... what's going on?" (And remember, an ounce of praise along with whatever your issues are with this individual will go a long way). If you're just looking for "proof" so you can take repercussive actions, whatever they may be... I again suggest the appropriate supervisor or manager deal with the issue. If the problem user is really spending all day playing computer games instead of working, it should show in his work. Make that the issue, not the "game-playing".

I apologize if this advice comes off as presumptuous, but treat your people like adults, address your issues like adults and things will be that much the better for it. Employee's don't react well to being (or feeling like) they are being spied on.

Just my .02 cents.

Are these locally-installed games? If yes, why are they there in the first place? If you suspect Web games, you can check your firewall logs to track what IPs his machine goes to fairly easily or setup a proxy server to also track/filter with more detail.