RE: how can I create an installation disc?
Yes. If you have the Win 7 ISO file, you can burn it into a bootable installation DVD.
Windows 7 ISO download........
https://www.w7forums.com/threads/official-windows-7-sp1-iso-image-downloads.12325/
How to burn iso to a bootable dvd..........
The Windows 7 USB/DVD Download Tool link to Microsoft is broken. Don't know why.
If you must, use another website. Just google for it.
RE : if I completly unistall windows and I would have nothing in my computer ... can I again install windows from this disc?
Yes. That's what an installation DVD is for,
provided you also have the valid product key for the corresponding Win 7 version. i.e. Win 7 Home Premium product key for Win 7 Home Premium, etc.