I don't know the exact reason for this.I am also seen this.SEP will get corrupted and most of the case there will be some virus also will be present.In my observation this is happening mostly to those PCs which is not getting regular updates.In some such cases we have to do a re imaging only.But may times following procedure helped me
Remove SEP
Delete following folders if present
C:\Program Files\Symantec
C:\Program Files\Symantec AntiVirus
C:\Program Files\Common Files\Symantec Shared
C:\Documents and Settings\All Users\Application Data\Symantec
Also delete following reg keys
HKLM\Software\symantec
HKCU\Software\symantec
Install SEP