I just don't want my more demanding games to suffer in the event that I accidently turned something off.
Generally it's best to limit the number of cores in the Windows Task Manager once a program is up and running. Press Ctlr+Shift+Esc to start it, locate the program on the Applications Tab, and right click it selecting Go To Processes. From there, right click the process and select Set Affinity (uncheck all be a single core).
This limits the selected task to a single core and allows all other processes to see the entire range.
Metzo has a nice write up on a utility (Imagecfg) that modifies the header information in an executable to limit the number of cores as well - I haven't used it on Windows 8, but I would imagine it would run just fine.
You can find his info below - (Search the page for Imagecfg) Metzomagic