Of course it's going to use 100% of the cpu time. That's what it's designed to do - USE PROCESSOR CLOCK CYCLES THAT AREN'T BEING USED BY OTHER PROGRAMS.
And then to allow other programs to take over quickly, because the science applications that it controls all run at a low level in the background.

If you didn't even know this basic fact about the programs, why did you even put it on your computer in the first place?

I dedice to deinstall Boinc too cause it's still running after I closed it. And 5 min. later its running again and than I cant close it from Taskbar - there is no menu.
So I have to start Taskmanager and kill all Boinc processes manually.

It helps to know what you are doing with your computer when you are installing programs. Reading what the installer gives as options is the best way to do things. Just pointing and clicking may be easy, but it can bring you trouble.

What I think you did was install BOINC as a service, so yes, if you then go File->Exit in BOINC Manager, BOINC will continue to run. It's one of the advantages of the service, it always runs in the background. Just as all those other services do in your Windows. Go take a look in Start->Run, type services.msc and click OK.

See those? That's what runs in the background. Most if not all even run before you log on to Windows. You can stop services here as well. Just double click on any of them and see the menu options that you have.

BOINC when installed as a service is very easy to stop. Just through that same Start->Run option, type net stop boinc and click OK. That stops the service. Want to run it again? Someone on Seti advises you to reboot the computer - also someone who apparently doesn't know how Windows works and has no iota of interest to ask - so let's forgo that "advise" and just instead go back to Start->Run and type net start boinc and click OK.

But in your specific case I would advise you to sell the computer and get a Playstation 3 or XBOX 360 or Nintendo Wii. Way easier to work with and it won't pester you with all these things you don't understand. It still does all your games, more of them even, while you can still surf the internet and use email.

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