Do you have your original Windows disk? You can put it in and see if the install / repair screen comes up. Since you already have an OS on it the Windows disk should give you a choice as to whether you want to re-install the OS or try to repair it. Sometimes there are hidden repair partitions on laptops, and there may be a way to access that information by looking up the troubleshooting and repair options for your specific model. Branded computers don't all have the same type of repair options as the typical hobbyists self built systems, so you really have read your manual to find out if there is possibly a function key you hold down while starting to access the repair data or some other procedure.