Hello, and welcome to PCHF!
Assuming you can get into the computer, insert your Windows CD into the drive, Press
Windows Key + R and type
sfc /scannow. Take note of the space between "sfc" and "/scannow." This should detect if any of your operating system files are corrupted and repair them if necessary.
Next, I want you to follow the instructions in the
PCHF Prework to make sure this issue was not caused by malware (and hey, even if it wasn't you might as well do it for the safe-and-sound feeling of knowing for sure that your computer is clean!).