There have been occasions in the past, where computers that do not get connected to the web to run Windows Update have had Sonar crashes, due to Cakewalk developers coding to changing maintenance levels of Windows.  Coding to current OS maintenance levels, or to reasonably current maintenance levels of the operating system is a normal occurrence, so this should be factored into computer maintenance plans. 
 
These kinds of crashes typically involve a user's computer being behind on maintenance with one or more C++ run-time redistributable routines, where Sonar is getting updated by a user, but the rest of their system is lagging behind on maintenance.  This gets code dependencies out of sync, and can cause problems.
 
Bob Bone
 

To everyone thanks again for your help !
Robert, I think my computer has been disabled concerning the automatic Windows updates; maybe does this cause problems ? (I have absolutely no idea of the "C++" things). Moreover, I had to download little programs like "Virtual Clone Drive", Java ...I ignore if this may cause conflicts (?)
And few hours ago, I installed Sampletank 3, and testing some stuff, and made a small and very "prudent"recording and guess what ?
Sonar crashes systematically and in various manner : 
One time, everything was stuck with the sound that froze as if I had pressed and held on a "stutter" effect button and I had to turn everything off  (sound card, controller, pc) and restart ! And other times, after closing Sonar, error messages
I confess that I have little fed up with Sonar concerning its Midi vsti management.
 

The stutter effect looks like audio interface drivers. Make sure you are on the latest including the firmware.

My advice in full....

1. Full backup
2. Turn on your firewall permanently and scan for viruses and spyware. If you've been on the internet at any stage (using browser or not )without a firewall you have been put at risk.
3. Run windows update several times including optional updates, that should sort out any c++ stuff anyway (although we have not seen any c++ related error so I'm scetptical ). It should also improve performance and reliability.
4. Install X3E. Make sure the plugins installed are on the latest patches. Perform a VST reset afterwards followed by a rescan.
5. Get the latest drivers for everything. Run 'Intel autoupdate' assuming your machine is Intel based. Make sure you are also on the latest firmware (esp your interface).
6. Reconsider about being always off the internet there really is no benefit nowadays. We seem to get a lot of people who have issues who are off the internet nowadays. You can always disable the network adapter whilst recording anyway.
7. Run chkdsk and defrag.
8. Consider disabling Intel speed step (assuming it is on).


It just might be that now is the time to reinstall windows on a clean partition.This really looks like a system config issue rather than Sonar. However I suspect it's a result of some system component not being up to date or a rogue or unwanted process. You could also check the windows event viewer for errors.

Also with that many DAW's running (I suspect you are running a lot of software) you might want to be looking at a deeper level. That is disabling process and services you don't need that will interfere.

I would recommend a sequential approach anyway as above.

Cheers.

Great replies folks, keeping them coming!!