When I read this, my first reaction was that it’s likely to be an out-of-memory situation.
On my old laptop with 2GB RAM, as soon as any app tried to use more than 700-800mb, the system would indeed start to lock up as Windows starts “swapping” (i.e. the performance killer). In such situations, it is practically impossible to do anything with the computer - heck, in some bad cases, the cursor even stops responding completely for up to 5 minutes until whatever it was has completed it’s business (in my case, it was usually Windows Update searching for updates). From what I can tell, it’s probably likely that the lockups occurred because Windows ended up swapping out system processes as well (!), including the display manager.
Anyways, back to your specific situation: sculpt + multires can result in high-memory usage situations. While the multires mesh in and of itself might not be a problem while working on it in the viewport (though it may be getting close), Blender will need to create a second copy of it (albeit, in a slightly different form) that the renderer uses. The combined load of both of these together probably requires more memory than you’ve got on your machine.
Since these freezes do occur and when they do they can become unable to be stopped, the best course of action is to be able to predict when this is likely to happen given your setup. Firstly, you’ll need to know how much RAM your computer uses post-startup (with just the background stuff). Based on that, you know the maximum “working memory” the computer has before it will start swapping. However, from experience, the actual threshold before it starts swapping is perhaps something around 50-75% of that maximum capacity.
Additional things you can do when you anticipate that you’ll be doing something memory intensive:
- kill anything non-essential, and then some more if you can kill them without bringing down the system (and/or the system lets you).
- try starting Blender from the command prompt instead, and issue Ctrl-C twice in the console you used to start Blender with when it starts locking up. This sometimes works (though not always, since you might not be able to even change windows by that point), but it should be slightly more responsive than the Blender GUI when things lock up