It's an accepted computing axiom that it's not a good thing to have more than one antivirus program installed at a time.
When you say "Norton" that does not help too much because "Norton" makes at least four consumer-level protection products, Norton AntiVirus (NAV), Norton Internet Security (NIS), Norton SystemWorks (NSW), and Norton 360 (N360). All of their consumer protection products have Norton AntiVirus as a component program, so if you want to run MSE you need to turn off the NAV part of any of the four Norton programs. Since MSE is an antivirus and antimalware program, if you have any of the other Nortons, you could still use the part of the Norton package that's not antivirus and antimalware along with MSE.
Buy Office 2007 Now, Get Office 2010 Free