A service in
manualmode can be launched by Windows when it's needed. For example this might be the case when another service depends on it. When you set such a service (which others depend on) to
disabled then the service depending on it will fail to start.
manual mode do not take any resources as long as they are not started by the system.
So if you want to make sure a specific service does never run, then just set it to
disabled but be aware that this might break other services or applications depending on the service. Some services are also "optional" to some applications. For example there is the "Telephony" service which is set to
manual in most Windows versions. Setting it to
disable seems to have no immediate effect on other services as most people do not use RAS and ICS services which depend on it. However recent Outlook versions will for example not display the "phone number assistant" when double-clicking into a phone number field when the service is disabled. If you set the service to
manual you will see the service running after this action. Each time you stop the Telephony service and adding another phone number using the assistance the service is started automatically. If you set it to disabled the assistance will simply not show up at all (no error message though).
So be careful when forcing services to
By the way there are also a couple of good web-pages out there describing the services in detail and their recommended state. Check BlackViper for example.
Some additional information and personal opinion on disabling services:
The guys at BlackViper also list the default settings. I personally do not even recommend to use the "safe" settings as these settings are intended to be used by people liking to disable as much as possible without breaking the system.
If you are sure you will never ever need the Telephony service, then you can also keep it at "manual" as it will never start then. The point is you usually never really know when you need a specific service. Taking the Telephony service as an example I think nobody would really know that the Telephony service is used to display the phone-number entry assistant in Outlook contact edits. Also I had to learn it the hard way. Once I've disabled all the services because I was thinking exactly the same way as you do: "Disable as much as possible to save some system resources". Then when I was using outlook I discovered that the assistant would not open (even without any error message) and I lost about 2 hour looking for the error (including re-installing Office) before I discovered the problem.
The service itself just uses a couple of kB of memory which is really a joke compared to today memory equipment in modern machines. If you do rigorously disable all services which you "think" you will not need, then you will probably save about 5 seconds of boot time. Compared to the hassle you could run in if you have to debug an issue because any application depends on a service you disabled but this application is not providing meaningful error messages, then it's not worth the effort. I would rather recommend buying a fast SSD; it makes bootup really MUCH faster without having to mess with services.
The only part I think you could manually adapt the service startup is for RAS services and similar things. Nowadays nobody is using dial up connections any more. These services are not needed. However also here some required services might depend on them and changing one of the services to disable might have unwanted side-effects.
So the bottom line is that the services and their startup are well-chosen by Microsoft (at least in Windows 7) and I would really recommend not to mess with it unless you really really know what you're doing.
Recently I had to fix a PC of a friend of mine. He was using some kind of tuneup tool (well, he was unable to tell me which one it was and he already uninstalled it). This tool disabled really all services which it "thought" are not needed. As a result on Windows 7 neither the Firewall, nor Windows Update nor Task Scheduler did start any more. The first two are evident for everybody that it's an important service. While many people think that they don't need the Task Scheduler service because they don't have any scheduled tasks. But this is a false assumption. Windows schedules a lot of background tasks. Including weekly defragmentation and similar optimization tasks. So after a couple of weeks you might end up with a system which is even slower than if you would have let Windows start a tiny scheduler service at each boot.
I am fine if somebody asks questions whether a SPECIFIC service can be disabled. Then the answer is usually "yes, under certain preconditions you can". But a general advise that a service can be just disabled cannot be given - else Microsoft would have disabled the service by default already.