I had a similar/same issue. LU would not run because I had the windows installer continually pop up trying to install SEPM. I had to repair the SEPM to get it working. I followed the steps in the link below.
You should only have to do the back up part of this page.
Preparing for Disaster Recovery.
http://www.symantec.com/business/support/index?page=content&id=TECH105658
Next, I ran a repair for both the SEP client and the SEP manager from Add/Remove programs or programs and features depending on what server you are running.