Okay, I think the easiest route would simply be to reinstall.
That said in light of your "no risk to Windows" sentiment I must make sure you understand there is NO risk free option here .. in fact to reinstall Ubuntu we absolutely WILL break Windows booting until Ubuntu is reinstalled so if at any point after deleting the Ubuntu partitions you find it impossible to continue and reinstall Ubuntu you'll also not (easily) be able to boot Windows.
The reason for his is because currently the GRUB bootloader config files (which are also chainloading the Windows bootloader) are contained on the Ubuntu partition we're going to need to delete prior to reinstalling. Windows will still be there but it will be unbootable until Ubuntu is reinstalled.
There's also the facts that ANY partition manipulation carries a (very) small risk of corruption, and there's the obvious scope for user mistakes such as deleting the wrong partition.
These risks are minimal, I'm not trying to scare you off but they DO exist, it would be wrong if I didn't make you aware of them
So, a bit of information gathering is in order before we continue.
Can you boot to the Ubuntu LiveUSB .. select "Try Ubuntu" NOT "Install Ubuntu" .. then once at the Ubuntu desktop open a terminal (Ctrl+Alt+T) and post the output from these commands
so we can get an idea of the current partition layout.