UserAssist is a small, free, portable tool which may be able to reveal the programs that are being used on a particular Windows 7 PC.
If you're curious about what your kids are doing on their computer, say, or how an employee might be spending their time on a company PC, then UserAssist may be able to help. Just run it, and you should see a lengthy list of exactly what software has been used on that system, along with a "Last used" date, various counters and other interesting information.
Why does Windows record all this? We're not entirely sure, but if you'd prefer it didn't then UserAssist allows you to erase the current list (Commands > Clear All) or stop it collecting data in future (Commands > Logging Disabled).
Alternatively, there are some system cleaning programs which will remove it for you. CCleaner, for instance, can wipe the list every time it's run (click Cleaner, choose the Windows tab, scroll to Advanced and check that User Assist History is checked).
Please note, if you'd like a version of the program that runs on earlier versions of Windows then try the UserAssist page on the author's site.