Did you completely uninstall Norton? If not, with or without a subscription, the program will still be running from start-up and conflicting with McAfee. To uninstall Norton you have to use their own uninstall tool, which you can download from here:
http://service1.symantec.com/SUPPORT/tsgeninfo .nsf/docid/2005033108162039
McAfee is known for slowing some PCs down. You might find that the best solution is to ditch it and switch to more reliable software. Nearly all of the 'regulars', here on AB's Technology section, use Zone Alarm Free Edition for their firewall, with AVG Free Edition as their anti-virus software. They usually top these up with something like Ad-Aware SE Personal Free edition or SpyBot Search & Destroy Free as their anti-spyware program.
Paying for security software is simply madness. Both Norton and McAfee have a reputation for memory-hogging, while the free stuff works much better.
Chris