I did as you told me and so I remembered also why I had turned off Dr.Watson some time ago. After restarting computer, I selected one of those video files that were leading me towards that error message, and got Windows Explorer freeze for some time, then that "Dr.Watson Postmortem Debugger encountered an error and needs to close" error message. I had to go to Task Manager - processes and manually "close" Dr.Watson. Windows seemed to restart as if I logged off, then I opened again Windows Explorer, selected another troubling file and the sequence repeated itself. The difference: this time there were two 'drwatson32.exe' running, the one that I thought I closed before and another one. Under no circumstances had I managed to end the first one, but I tried to end the second and Windows restarted again, in same conditions.
A while ago, I guess that would mean 1.5 months I had the same problem and found out via Internet (I think even on this site) that the problem should be eliminated once having Dr.Watson turned off. Some time I had no more problems, then... this thing I related in my first post drove me crazy and had to effectively ask for assistance.
Thank you and I have a little request: I'll be here in front of my computer until you have to quit for tonight, so please, when you think you're posting your last reply tonight, let me know, so that I could turn my computer off. But I repeat, as long as you have time to help me I won't move, it's far too important to me having this solved out without having to format my HDD and reinstall XP.