Why is it that when I try and install Kubuntu (14,15) on a PC with Win 7 installed, Kubuntu makes Windows unreachable

Because you installed it incorrectly is the most likely explanation. Not having any details on how you installed, what your drive/partition layout is it it impossible to give any more details. Did you just "try" to install Kubuntu or did you actually install it? Can you boot it from the hard drive? If so, open a terminal and run the following command and post the output here for a start: sudo fdisk -l(That is a Lower Case Letter L in the command) If you can't boot the installed Kubuntu, boot the installation medium and run the command.

Thank you for your post.
A bit of history:
I have win 7 (pre-installed) on an HP Pavillion. I first tried to install Kubuntu 15, which created some problems due to in the boot systems incompatibility; I recovered with Win 7 recovery tool. Then I installed Kubuntu 14, which succeeded but then Windows was no more visible. I recovered Windows once more. After various attempts which I will not deal with, unexpectedly, after another Kubuntu 14 installation, now I Have both Kubuntu and Win in the boot menu.
I still do not understand why it did not work the first time.
No I have the following partitions:
/dev/sda1 NTFS(was once the boot partition)
/dev/sda2 NTFS(now the boot partition for win)
/dev/sda3 ext4 /
/dev/sda3 swap
/dev/sda4 NTFS