I have installed Sonar X2a Studio and Producer Demo, X1 Le as starterkit too. But now all is damaged. When I start one of the programs, the system freezes, not only Sonar but the whole computersystem gets very slow and the GUI is nearly damaged. So I have tried everything. uninstalled all Sonar-programs and installed it again. Nothing has changed. So I tried it again and deleted all files and directories of Cakewalk. I even deleted all entries of Cakewalk  in the registry, which I found. But even that doesn´t solve the problem. 

A suggestion if your Windows is not up to date.. Patch up to latest Windows update including your graphic card drivers (may have to download separately, or most recent driver have utility to check for update).

I'm sure others have more suggestion to make your system as solid first before further Sonar installation.

I have used SONAR X2 for about three days without problems. Then something mysterious happened, I don´t know what. And since then nothing seems to work anymore. Perhaps not the Sonar-programs are responsible, but the Windows-libraries, which are used by Sonar. 

you might wnt to run a virus scan just to rule out any foul play.
use this:
http://www.malwarebytes.org/

this is better than anything your anti virus has for removing malicious files.
if you've never used malwarebytes before, well you've found yourself a new virus removal software.

Other thing to try is to load Sonar in safe mode.  If that loads ok and you can get the midi preferences setup ok, quit and restart.  That *should* reset any weirdness.

I have checked my system with a virusscanner and found a malware, but that had nothing to do with the Sonar-problem. Then I started in safemode and everything was alright. There were no audio- or mididriver. I checked it and pressed OK. But when I started again in nornal-mode, the problem was the same as before.

Ok, so booting in safe mode is ok.  That's a good start.  At what point is it hanging in 'normal' mode?  When it first starts you should get the new/existing song dialogue box.  Can you start a new song using the default template, or does it hang before even then?

I would disconnect any esxernal midi stuff (keyboards, control surfaces) initially.  It may be your soundcard drivers are giving problems here.  Can you post your full pc spec (including o/s, 64/32bit, soundcard, system resources etc?

When I start Sonar, there is the dialogue box and I can choose anything. But - no matter, what I choose  - the project is opened and soon the graphic freezes. All is very slowly, the computer nearly doesn´react anymore. The whole graphic-system seems to collaps  I have to kill the process in the process-manager. My system is : Win7 Home 64 on a Acer Travelmate 5742Z laptop with 2.13 GHz Dual -Intelprocessor, 8Gb ram , internal soundcard and Behringer USB UCA 202 Soundcard,
 M-Audio Oxygen 49-Midi-Keyboard,  32-bit-Version of Sonar. 
I have disabled the keyboard- and audiodriver, but that didn´t help. 

One last Sonar specific - do you have any 3rd party VST/VSTi installed?  If so get them out of the way so all you have is a vanilla version of Sonar.  Then run safe mode, create a new project using the default template and confirm that is still giving problems.  You should be able to run multiple versions of Sonar, but in my experience I've found it best to remove all traces of competing versions when installing a new one.  Even if that means losing a previous add-on.

Normally a duff driver could well result in program freezes or crashes.  Do you have an oboard soundcard such as a realtek?  If so you could try removing your Behringer one and running with just the realtek.  Don't worry about latency here - we're just interested in Sonar being relatively stable.  Alternatively, disable the onboard sound card and make sure you have the Behringer installed correctly and try the same thing.

After that you may be in to memtest and hdd checking for the rest of the hardware.  People have crashes with Sonar but generally not this consistently or immediately.

If all that still fails, you might want to try installing another DAW (Reaper or whatever) just to see if the whole DAW thing will work on your system at all.  Your specs suggest you should be fine imo.

Anyone else have ideas?

I have a lot of VSTs and VSTIs. But it didn´t help to deactivate all of them.
I have deinstalled the drivers for the onboard-soundcard and the extern soundcard and disconnect all devices, but that didn´t help too.

With other DAWs (Studio One) there are no problems. What I really don´t understand is, that the whole graphic is corrupted, when I load a project in Sonar. Not only the graphic in Sonar, but the graphic-system at all doesnt work anymore. Can there be a problem with the graphic-driver?