Sorry to hear about your problem, Jakeshiraz.
Have you checked the boot order in BIOS? If you have then try to see if disabling Secure Boot (under UEFI, also in BIOS) helps.
You can also try this: https://help.ubuntu.com/community/OS-Uninstaller
If you installed Zorin with EFI enabled and the BIOS has a Zorin EFI key it could be protecting the system. The simplest solution would be to unplug the power lead from PC, take the cover off and earth yourself by touching the metal cover you have removed or a heating radiator then remove the lithium battery from the motherboard, wait 30 seconds then put the battery back in. This will (should!) reset the BIOS to default. You may want to consider downloading GParted Live, boot off that to delete all your Zorin partitions then create one big NTFS partition ready to take Windows 10. If Windows 10 was activated on last install there is no need to start with a Windows 7 install first as M$ spyware keeps all your hardware ID in its Redmond vault.
So pleased for the replies from you all which is much appreciated. I am very sorry for the late update as I did not know anyone replied I thought I would get notifications in my email box!!!
I will give you feedback after implementing the advice.
SIDEBAR -- based upon the computer's age -- is it possible the age of the BIOS is causing a problem? As in it may need to be updated before the user can proceed?
Sorry for the headaches you're going thru. I have no answer, and these links may not help, but I consider Britec a Guru's Guru. Perhaps there's something below which may help --
I'm offering these in case there was a step or tip you missed.
Many thanks for the feedback. I must apologise to people for the delay in replying as I do not have notification as a default setting so unaware when replies come in.
The steps I am at:
I have managed to install Windows XP (vintage edition II) from a disk. For some reason a USB won’t take it despite the fact I am booting correctly, the USB is created correctly and the port works. I know this because I have installed Zorin 100,000 times using it.
Windows just sticks on the logo if using USB to install W7. No further progress.
Legacy/UEFI mode cannot be found in the Bios of the computer. Compaq DC7800p.
So stuck. I will try solutions from what has been suggested.
I still recommend you try the three links.
If your problem is related to any residual effect of Zorin, one of those links will clean out any traces. Then you can eliminate your older Zorin install as being a cause.
If your problem is not related to any residual effect of Zorin, then the topic is purely a Windows issue, thus out of scope of this forum.