Thank you for your answers and sorry for my poorly explained problem. I'll try to be a bit more specific in the future.
As I said before I tried to clean install Windows 7 on my old Computer.I wanted completely clean Computer. I started it by booting from a retail CD/DVD using Sandberg external DVD drive. It was connected in 2 USB ports in the front panel of my Computer. I've had some problems whit those USB ports in the past. For example sometimes my Computer didn't find USB devices connected to those ports.
Windows installation began normally. It started to load files. But after the loading finished I got an error message:
``Windows failed to start. A recent hardware or software change might be the cause. To fix the problem.
(Edit: No new hardware or software installed)
1. Insert Windows installation disc and restart your Computer.
2. Choose language. Then NExt.
3. Click ``Repair your computer``
If you do not have this disc, contact your system administrator or Computer manufacturer for assistance.
I don't remember all the files that came in the next part, but first one was CI.dll. Everytime I tried to start Windows again the file was different. For example \Windows\system32\config\system
In the info part there was ``Windows failed to load because the system registry file is missing, or corrupt.
Today I tried to recreate the error message so I could write the exact error message here. But as I turned the Computer on all I got was the motherboards brand on the screen and after that an message: An operating system wasn't found. Try disconnecting any drives that don't contain an operating system. Press Ctrl+Alt+Del to restart.
As you guys adviced I plugged the old HHD on another Computer and it seems to work properly. I used Windows CHKDSK and WMIC. Both said that the disks status is OK. I also tried the SeaTools as joseibarra suggested whit same results.
Thank you again for your answers and help !
Edited by ComputerAmateur, 09 November 2017 - 11:43 AM.