Can't go into stand by

Would appreciate anyones help. I have windows XP. When I hit start >turn off computer >stand by, the pop up disappears and I go right back to the desktop. The "restart" and "turn off" functions work but not the "stand by" function. I have already checked control panel >power options and all those settings work and are fine. My computer will eventually go into "stand by" and "hibernate" per the settings. I just want to be able to "click on" the stand by button and do it manually like I used to be able to do.

 

Has standby worked previously? Have you reinstalled the OS for any reason? If you have your XP cd to hand put it in the PC then go to 'start', 'run', and type sfc /scannow then hit enter. This will check all protected windows files for errors/locations

 

Thank You for responding. To answer your questions. It was working before and no I did not reinstall the OS. I tried your suggestion. I played the XP cd and typed sfc/scannow and I got a message that windows did not recognize that command. In the past I remember there being a repair file function on the XP cd but now it appears I cannot access that because of installing SP2. As a side note I never installed SP3.

Is it possible I have somehow corrupted the OS?

 

Make sure you have a space between the "sfc" and the "/scannow" when typing in "sfc /scannow"

 

Again I really appreciate the help!
OK I did as you instructed. I left a space between the "sfc" and the " /scannow". Windows ran the program completely. The only glitch was a constant pop-up asking me to re-insert the cd when it was already in the deck and running. So it ran the whole process. I watched the bar move all the way from left to right. After it was finished Nothing. It disappeared from the screen like Disc Cleanup does when it is finished. There was no indication that something was wrong. Do I assume that everything is OK? I still cannot go into standby.