Try running the system file checker. To do this simply go to the Run box on the Start Menu and type in:
sfc /scannow
Then click OK.
If there are no problems the entire check will complete but if not it will ask you to insert the installation disk,
I understand you do not have the installation disk but there is a work around. Its a matter of checking to see if there is a CD-ROM image is on your machine. If so redirect it to that.