Thank you DavidJPotter for your input. Most probably it is me who sucks doing video editing rather than the program I have been using. I have done some video editing with the videos I have been filming (my son playing, birthdays, family parties and stuff like that) in HD mp4 format with Serif MoviePlus and before that using other formats (using different cameras), facing no problem at all. Anyway, if the problem would be due to the program I use, maybe whoever effectively did some video montage of Home Designer´s walkthroughs successfully and painlessly might comment on which program is he or she using.
And thanks Mick (KBird1), as you said, probably I am using the wrong settings. The thing is that I tried several different settings and I haven’t find any yet that would work fine. MoviePlus is quite simple an intuitive to work with and it offers plenty of possible settings for recording or converting video and as I said, I have done some video editing before in a quite straightforward manner with good results. I am not being able to do so with my walkthroughs (which are fine by themselves).
I also tried with Movie Maker (which is for free and much simpler and offers less setting options than MoviePlus), speculating that maybe the problem had to do with an outdated avi codec (or whatever). I believe avi is a format developed or used by Microsoft as Movie Maker is (or maybe I am wrong) so I downloaded the latest version of Movie Maker. But I got the same unsatisfying results as with MoviePlus.