You're right, it is difficult to determine these, but what I do...
Generally I don't mess with Services unless I am absolutely sure. Changing the settings on Services can have unfortunate effects on the running of a system. However, there are resources on the internet that can help you decide which 'default' services can be adjusted. The one I use is Black Viper. Scroll down the page for the list of default and/or common services. Note, that this is for Vista with Service Pack 2 installed.
It is then a question of working down the list and comparing that with your own list. Read carefully, and "Expect differences"! If you want to change settings for any Services, just double click the entry in your Services console to display the Properties dialog.
With Startup files there is no such easily formatted list, because each Vista system is different. I don't know which Startup list you have obtained, but the one we use is the "System Configuration Editor" or commonly known as MSCONFIG. We get there from the Run command in the Start menu. If you do not have a Run command shortcut pinned to your Start menu, use the Search box there and search for Run, then open the Run in the list.
In the Run dialog box, type in msconfig and click OK, then in the new window, look at the Startup tab.
The general rules are;
1] Whenever you install software, check the install procedure carefully. Many of today's software are desperate for you to use their application exclusively and so they want it 'loaded at startup' and ready for you to use. generally that is not required as you can simply open the software when you need it. Look for any options to "Load at Startup" and if you don't want that, remove the option.
2] What we don't need to 'load at startup'. Instant Messengers, browsers, "Real Player", "Quicktime", Adobe, Updaters like Google Update, Java, iTunes, any software that you can simply run from a Desktop icon or Start menu shortcut. Anything that looks strange.
In that Startup tab the column widths may be too narrow to see the full entries. Grab the column header divider and drag to the right to see more. If you are unsure what an entry is, use Google to search the entry details.
Please note. You don't say which "Services" list you are looking at. If it is the one in msconfig, please do not use that. Instead, use the Services Console, available by using that same Run command, typing in services.msc and clicking OK.
I hope that helps.
There must be many programs that are not essential to run at start up.
I have made a list of those listed under Start Up
and a separate list of those appearing under Services.
Unfortunately, the detail explanations are a total mystery to me;
how do I determine which have to be running at start up and which can safely be taken off that list?
I have a Dell XPS 420 running Vista Home edition