I ran into this the other day. I had to remove SEP11 and do some manual cleaning as well. After you remove the client and live update do the following:
Delete the DWH files from temp.
Delete all Symantec Folders in Program Files.
Delete the Symantec Folders under C:\Documents and Settings\All Users\Application Data
Install what ever client you want back on and the issue is gone.
From what I could tell the issue is that when you get new definitions, SEP scans the quarentined files to see if any can be repaired. When SEP unzips the quarentined files, they are put in the temp folder. Then of course they are scaned by the Auto protection and the Def. scan which causes and endless loop or quarentined files.