Many thanks for all the good advices. It seems that the burner has started working properly again. I checked it with fixit, and then I cleaned the lens with a wet screen-cleaning tissue, and things started going smoothly again. I tested it just once, but I hope that the issue is permanently fixed.
But I think that part of the problem is that this machine is nearly 2 years old, and I have burned already lots and lots of CD's. I take extremely great care of the laptop, however: I run disc cleanups daily, and I defragment the c: and d: units regularly with the standard windows defragmenter and also with the excellent Puran Defrag, and of course I take extreme care not to get any virus. But at some point perhaps all this will not be enough, for time passes...
How long do you think a machine like mine can run acceptably?