Use the chart in the link below and match the "safe" column.. If you need more removed, then try the "tweaked" column.

As to Norton disabling Windows Defender, it makes sense... Since Norton has an antispyware program of it's own, two "real time" programs running at the same time can cause conflicts.. Whether you want to keep Norton installed then becomes a personal choice..Accessing important sections of the operating system is essential and if you can't temporarily disable Norton's protection so you are able to make changes, I'm not sure I would keep the program.. But that's up to you..

As to accessing the startup programs, have you tried running "msconfig" (without the quotes) from the "Run" line?

Hope this helps.