OpioniatedPotato :
So, my new pc had windows 7 oem on it untill i changed to linux ubuntu how can i go back to windows 7? (i still have the installation disk)
Also could you try to explain it simply lol.
I assume you bought that PC with windows 7 preinstalled.
In that case somewhere on pc case should be sticker with your windows 7 product key.
With that and installation disk you can reinstall windows 7.
Just put disk into your optical drive (dvd drive I guess) and try to reboot. If you boot back to your classic system you will have to change boot priority in bios.
To do that, you have to press DEL key or F12 (depending on bios, it may be some other key also but it should write you hint on initial screen).
When you get into bios look for boot priority or device priority.
Your HDD/SSD will be probably listed as 1st. Change it to your optical drive.
After that save these settings and exit bios. It will reboot again and it should start installation of windows 7 just follow instructions on screen. It will ask for product key from that sticker on pc case so it's good idea to have write it down before you start installation. It is often located at the back of pc case.
Remember to change boot priority back to HDD after installation ends.
Before you reinstall remember to backup all important data you have.