Hello there I know a little bit about media center so lets give a couple things a run
1) open termianl and enter in
sudo apt-get update
apt-cache policy xbmc
Does it say that it is installed ?
Next if it is What is the version Number and Where is the Software coming form ?
If you could paste that back for us to see that would help out a bunch.
~$ apt-cache policy xbmc
*** 2:11.0~git20120510.82388d5-1ubuntu1 0
500 http://us.archive.ubuntu.com/ubuntu/ quantal/universe amd64 Packages
Now you can do that for all of the packages that xbmc reqires to see if there are also installed o removed.
How do we know what packages are attached to xbmc
with this command
You should get something kinda like this.
apt-cache show xbmc | awk '/Depends:/,/Size:/'
Let me try to explain what this is.
Depends: xbmc-bin (>= 2:11.0~git20120510.82388d5-1ubuntu1), xbmc-bin (<< 2:11.0~git20120510.82388d5-1ubuntu1.1~), mesa-utils, x11-utils, ttf-liberation, ttf-dejavu-core, python-imaging, python, python-support (>= 0.90.0)
Breaks: xbmc-data (<< 2:11.0~git20111222.22ad8e4), xbmc-skin-confluence (<< 2:11.0~git20111222.22ad8e4), xbmc-standalone (<< 2:11.0~git20111222.22ad8e4)
If you look at this you will see that there are a few important parts,
Depends: This means that the packages (xbmc) 100% needs all the other packages that are under this KEYWORD of Depends:
Recommends: Indicates what other packages are recommended when (in this case, xbmc) is installed
Breaks: Indicates what packages will conflict with this package. Conflicting packages cannot be installed on the same computer.
If you have removed all of the packages that THE OLD xbmc was using and you have also purged and removed the packages, i.e.
then you should be able to install re-install xbmc
sudo apt-get --purge remove <package>
Let us know how it works out