You have seen many recommendations already and I don't know much about your Grandma other than she likes KDE on debian.
Personally I am not a fan of KDE because i find it has more issues than other window managers and there are some other quirks I don't like.
I won't go into the details as it's mostly preference than anything else.
If you are set on KDE because that's what granny is familiar with there are only two distro's I have seen that I like. For someone that wants something similar to windows with a smaller learning curve I can only recommend PClinixOS. I did'nt use to like them but recently it has become more stable and is starting to impress me. It is based on mandrivia so you cannot use debian or ubunto repositories unfortunatley.
It also has a rolling release cycle so you dont have to reinstall everytime there is a new version.
Personally I use linux mint with the cinnamon interface. It is based off of the new version of gnome but has a more standard interface which would be relatively easy for a windows user to get a hang of. Both distros have some unique features I have not seen in many other places.
Before deciding which distro to install for you Granny I recommend you try each distro that has been recommended so you can get a feel for which one she will like.
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You have seen many recommendations already and I don't know much about your Grandma other than she likes KDE on debian. Personally I am not a fan of KDE because i find it has more issues than other window managers and there are some other quirks I don't like. I won't go into the details as it's mostly preference than anything else. If you are set on KDE because that's what granny is familiar with there are only two distro's I have seen that I like. For someone that wants something similar to windows with a smaller learning curve I can only recommend PClinixOS. I did'nt use to like them but recently it has become more stable and is starting to impress me. It is based on mandrivia so you cannot use debian or ubunto repositories unfortunatley. It also has a rolling release cycle so you dont have to reinstall everytime there is a new version. Personally I use linux mint with the cinnamon interface. It is based off of the new version of gnome but has a more standard interface which would be relatively easy for a windows user to get a hang of. Both distros have some unique features I have not seen in many other places. Before deciding which distro to install for you Granny I recommend you try each distro that has been recommended so you can get a feel for which one she will like.