Update 2 (sorry for frequent posting): I have the answer to your problem (This worked for me, I don't know if it works for the regular Internet Security too).
First, uninstall EVERY Symantex program you've got. After you are CLEAN of those programs, do the following:
1. Delete leftover files. These are found in these locations:
A. C:\Program Files\Symantec (this could already be removed)
A. C:\Program Files\Common Files. Delet the Symantex Shared map.
C. C:\Documents and Settings\All Users\Application Data. (If Application Data isn't visible, enable 'view hidden files in Map options). Delete the Symantec map COMPLETELY.
D. Each account that the programs have been used on also have files (check all of them). See C:\Documents and Settings\(enter user-name here)\Application Data. Delete the Symamtec map.
E. ****These are register deletes. It is advisable to make a backup before you do this****
Open the Regedit (type Regedit in the Execute window(or whatever that is in English, I have a Dutch version). (Or go to Task Manager, New Task, and typ Regedit).
Now go to HKEY_LOCAL_MACHINE\SOFTWARE. Delete the Symantec map.
Go to HKEY_CURRENT_USER\SOFTWARE and delete the Symantec map here.
I would do a reboot at this point. After that, install Internet Security. This time, the registration window will appear.