You can not run
System File Checker for Windows 7 on Windows XP because the system files are different. That may be why it isn't working for you.
Safe Mode Command Prompt will load, you can try it from there. Or use an alternate Windows XP boot media then run
sfc /scannow /offbootdir=d:\ /offwindir=d:\windows from the command prompt. Here are more detailed instructions. You will most likely need the Windows XP CD in the CD drive when you run this.
Note: Depending on how your computer is setup, the Command Prompt, when used from outside of Windows, doesn't always assign drive letters in the same way that you see them from inside Windows. In other words, Windows might be at C:\Windows when you're using it, but D:\Windows from the Command Prompt in System Recovery Options.