I know this sort of thing can be very frustrating... Your subject mentioned
a bad repository; is it correct to assume this is an error that was given
after trying to install some software? Often if something happens during
install or download, it can be fixed with synaptic or terminal commands
similar to "sudo apt-get -f install " where you
replace" " with the name of the binary you are trying to
install... alternatively "sudo apt-get --purge remove "
(adding the "purge" option should clear all config files as well as removing
install files of a given program)... but the best solution can actually be
very different depending on the senario.

What may help others help you the most would be to include a link to the
directions you were following, as well as pointing out at which point during
the install you got an error and what specifically the error was.