Symantec slow computers

Hi, i have installed symantec 11.0.2000.1567 on about 100 computers sme of them only have 256 MB memory and 512 MB  Windows  XP Professional SP2, now i have done everything to speed up the computers but no luck,we aslo have Diskeeper 2007 11.0.709.0 running automatically in the background to best perfomance i have done defragment,disk cleanup,check disk, have disable some services,msconfig for start up programns,Disable Indexing Service, Show shadows under menus,Show shadows under mouse pointer, Show translucent selection rectangle
 Smooth edges of screen fonts, Use drop shadows for icons labels on the desktop
 Use visual styles on windows and buttons

in Symantec:
Change Settings | Configure Antivirus and AntiSpyware Protection | File System Auto-Protect
untick Scan files on network drives
untick if your machines have no floppy drive ,Change Settings | Client Management | Schedules Updates,Change the Frequency to daily : and select a time ( 10 pm ),Advanced | Scan when a file is modified
no luck still slow also to log in to windows takes like 10 minutes to the computer can be operated, i dont know if getting rid of symantec is my solution, and if i do wich antivirus won't slow my pc like symantec  i know i need more memory a least 512Mb but the users basically use the internet only , thanks