Go to Start then Help and select the Index tab, and type in system it should auto take you to a list then click on the system file checker select display and then click run now.
If it still doesn't work, you can try doing a Find Files option type in sfc.* (* is a wild card so will search for all files beginning sfc.) if it is not in the windows/system folder I would guess you have a problem with windows and need to reinstall after first backing up your data.