The problem with shutdown after x minutes of no activity is that it will close applications and possibly lose work. So, if you have a doc file open and walk away to take a dump or get distracted by a phone call the computer could wipe out your work.
Making the computer go to sleep (or hibernate) is much safer for your files. Sleep works awesome in Windows 7 for me and I'm yet to use hibernate.
the software i posted also has the options to sleep and hibernate as well as shutdown