Thanks to both of you. Neither of the suggestions worked but they gave me the right leads and I have now solved the problem.
1) Use videocut (GUI-only), or
2) Use SlickSlice (CLI-only).
Installation:
1) sudo apt-get install videocut, or
2) Go to
http://slickslice.sourceforge.net/
2.1) Download ImageMagick and extract
2.2) ./configure
2.3) make
2.4) sudo make install
2.5) if it complain about perl, then write "sudo ln -s libperl.so.5.10 libperl.so" and goto 2.2
2.6) sudo apt-get install mplayer
2.7) Download SlickSlice
2.8) chmod +x slickslice*
Done