This is just because of old version of symantec residing on your computer. Even when while unstalling it will give the same error. Follow below steps :
1. In Add or Remove Programs select Symantec Antivirus, and then click Change.
2. Select Remove.
Symantec will be uninstalled. :)
3. Delete all symantec related folders from Program Files, Common Files and Documents And Settings.
4. Try the installation again.
5. It will work. Atleast it worked for me.