my company just gave me a dell precision M4500 with ubuntu on it. I want to move to windows 7 as I know it is pre-installed (i have the serial number) but how can i do it as I don't have any dvd to boot ?
If you can't get a Dell OEM WIndows 7 DVD from your company, they're available on EBay for a few dollars.
Neither Microsoft nor Dell supplies WIndows 7 media any longer - so that's the only option.
You can use the Windows 7 Home Premium Product Key to Clean Install Windows 10 Home:
Alternatively you can Download the Dell Windows 7 Professional Skylake Reinstallation .iso and delete the EI.cfg file from the Bootable USB to clean install Windows 7 Home Premium (this is a bit convoluted):