It seems like from your side there is no fault.But you can give a try to below instruction..
Sometimes copying files from one place to another place or downloading from the download manager by pausing or unpausing.. break the original file(a.k.a accidental corruption)..so always compare the md5sum values.
To do so open your terminal:
1) change directory to the location of your file
2) Then type the following command
md5sum should then print out a single line after calculating the hash:
3) Now go to this link UBUNTU HASHES..
This page contains all of the md5 hashes for the different versions of Ubuntu,Kubuntu, Edubuntu, Xubuntu and Lubuntu.
4) copy the hash value from the terminal and search in the browser by pressing (ctrl+F)
if there is no match ..
then surely your xubuntu file is corrupted.consider downloading the new iso
xubuntu file is fine. Error might be related to disk partition.