This is commmon asked question regarding IE. Windows is actually using IE components too and that's one reason why IE should be kept up-to-date too whether it's used or not. If Firefox isn't used then I recommend to uninstall it.Is it okay to not update IE or Firefox, since I do not use IE anymore and I don't intend to upgrade FF to above 6, since last time I upgrade to FF7 it has error? Currently I only use Chrome.
Also, I noticed that my Norton Internet Security has been blocking OTL.exe saying that it has medium threat. I think Norton has in fact deleted OTL.exe from desktop once and I had to download a new OTL, should I be concern about this?
That's false positive and should be ignored.
Other medium threat levels identified by NIS are:C:\Windows\System32\SVCHOST.EXE , C:\Windows\System32\SERVICES.EXE , MBAM.EXE, GOOGLEUPDATE.EXE. Should I be worried about it?
I don't know Norton software myself but I suspect those are just info messages and not actually infections. Logs looked ok and unless there're any symptoms present I wouldn't worry.