su - Authentication failure - reinstall fedora ?

[Angelo Moreschini]

Hi,
after that I installed fedora 20, I am not able to get the login as su.
When I try to sign me as super user, I always get the answer
"Authentication failure".

I don't think that I really type wrong the password that I set at install
time:* I typed it twice*.
Rather I think that they are some funny characters inside it.

So I lost the hope to try to guess the password and I think to re install
Fedora.., but I have a problem.

My computer is dual boot with Windows XP and I am not sure how uninstall
Grub (how to manage the MBR when to the computer start) before to uninstall
Fedora...
Some time ago I tried to fix this problem using Windows in recovery mode
(command fixboot) and, at last, I had to reinstall also Windows XP (further
that Fedora)..

Can I have an advice how  to arrange the dual boot on my computer, before
to uninstall Fedora??

Thank you
regards

Angelo
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