I lost my windows cd I downloaded a copy from the internet. The guy who shared it said it's original without any modifications. However I get weird actions like I can't use mouse clicks or I can't type a letter on the keyboard, windows get closed. So how can I tell if there's a backdoor or mot?
If you bought an original copy of Windows and lost the CD you should be able to contact Microsoft with your installation code and ask for a replacement.
Don't download an illegal copy of a software from the Internet -- you're looking for trouble. It might very likely be infected with trojans, viruses, or any other kind of malware. Sure, you could run an AV on it, but since it's the OS itself that's pirated, I wouldn't trust its results.