From the message it's a bit difficult to tell if it's the apt database (if it is, how to fix it is described below) or the actual python download. Try to figure out by starting an xterm, then log in as root, and do

apt-get install python2.5-runtime -su

(the -s makes it do nothing, but it should tell you of any problems).
If it looks bad, try dpkg --purge python2.5-runtime

If you get any other kind of message (better post it here in case it isn't clear) then it could be the apt database:
Check /var/lib/dpkg/status
There is a /var/lib/dpkg/status-old as well. That's your backup. If you have becomeroot installed, or any other means of logging in as root, try the following:

cd /var/lib/dpkg
cp -p status status.bad
cp status-old status

then try refreshing your application list from the application manager. Oh, and do a 'df' first to check that your file system isn't full or nearly full (the filesystem you're looking for is the one which says just '/' in the rightmost column).