[PC Dumbo]

I've been working on my college assignments on MS word 2007, which is
completely new to me, as I have been used to using MS Office/Word 2000.
Anyway I seem to have done something which has caused a blue
filter/highlighter of some sort to appear every time I click on my work,
which then makes adding or editing it pretty impossible! Can anyone throw
some light on what I have done, and how to get rid of it, and what is the
blue filter/highlighter for? Please Help!!!!