Hi Ghislaine,
This is Mike from the Norton Authorized Support Team.
Contrary to what other users may say about Norton 360 version 3, under normal operation, and assuming you have enough resources available, Norton 360 version 3.X uses very little system resources and should not slow down your PC. Also, Norton 360 contains the same features of Norton Internet Security, and also includes a Backup & Restore function and system optimization features.
Since you mention that you had Norton Internet Security installed and then installed Norton 360, there is the possibility that your installation of Norton 360 is corrupted.
To resolve this issue, please carefully follow the instructions below to properly remove and reinstall Norton 360.
1. Make note of your current Activation Key.
2. Click on the following link to download the Norton Removal Tool:
Norton Removal Tool and Instructions
3. After you run the tool, please restart your computer. Then log in to Windows again and run the removal tool again. Restart your computer after it is finished running the second time as well.
4. Click on the link below and then click on the "Download" button under Norton 360 to download the latest installer.
Download the latest installer for Norton 360
5. If prompted during the installation, enter the Activation Key you saved in Step # 1.
6. When the installation is complete, launch Norton 360, mouse-over the "PC Security" icon …