Disable the Medibuntu source in Package Manager and also the two that
refer to 'woody' as these are now too old and the repositories are not
on-line any more :-)

BTW - from the 'woody' references I assume that you've upgraded
several times so a word of warning about going from 10.10 to 11.04 -
upgrades are risky at the best of times but the major changes that
11.04 has introduced make it even riskier, especially on a system
that's already been upgraded from earlier versions. Chances are that
upgrading to 11.04 is going to go badly for you so be prepared for it
all to go horribly pear shaped !!