Classnp.sys is very important part of windows operating system files that is driver for SCGI devices. It is blue screen of death error and this windows boot error occurs when you start your system in safe mode and booting your system from the DVD.

Windows classpnp.sys mostly cause by a lot of reasons, they are virus affected on your system heavily, corrupted files or folders, system files is missing or damaged, a device driver files is damaged or missing, outdated or incompatible driver installed, improper or accidentally system shut down and this error occur on your system for the most common reason they are invalid/ corrupted registry setting or files or entry.

It is not easy to solve windows boot error classpnp.sys manually, because of unknown problem area in the whole system. So, you can copy your important files from another pc and past on the same location of your system.

• You can also restarting your windows in debugging mode:

1. First restart your system and press on F8 button.
2. To get a list of boot options, which include in safe mode, last known good configuration and etc.
3. After this windows will load in debugging mode
4. And then you can restart the computer.
5. Now it will start up in normal mode.

• Run or check disk from windows recovery environment:

1. Access Windows Recovery command (as stated in the link below)
2. Run Check disk to fix any bad sectors on the hard drive.
3. At the command prompt type chkdsk /f
4. And then click enter.
5. Now, type in chkdsk /r 
6. And then click enter.

You can also use third party repair tool or RegCure pro software. This is one of the best system optimizer software that cleans all the error from computer easily. And it also repairs all windows error files and removes associate errors including classpnp.sys boot error.