Hi,
I agree with Curlyben; get rid of Norton.
You might already know how, but just in case:
Go to Control Panel, then Add/Remove Programs. Left click on Norton to highlight, then click on Remove; just follow the windows' instructions.
There usually are problems trying to run two Antivirus Programs at the same time.
This might account for the slower computer.
I switch from Norton to AVG about 3 yrs ago or so; and have NOT had any virus get passed it!! It has caught many, mainly through emails. It checks both incomine and outgoing email.
You might also consider a great, free cleaner, works better than Disk Cleanup, from: http:
www.ccleaner.com. Just two clicks gets rid of junk files, temp folders, some never used Registry entries, etc.
Also, AVG allows downloads, almost daily of new Antivirus definitions, one at a time sometime; instead of waiting to "compile" many at the same time.
They are available as soon as they are ready to be released!
Best of luck.