before unticking you can look up the start up items click here see 'full list' to find out what they are.
if you really want to delete from the reg then most are found in:
HKEY_CURRENT_USER/software/microsoft/windows/currentversion/run and runonce
click start/run and type regedit click ok.
navigate to the above and right click and delete the entries you want here. be very careful only to delete the entries you are sure are safe to remove and nothing else.
to back up the reg before trying this open regedit and click on 'my computer' to save the entire registry or highlight the entry you are going to remove and click 'file/export' and save the reg to a location of your choice. you can restore this by clicking on the .reg file you created if you need to.