@arturox: The more conversion methods fail to produce a result you like, the more it sounds like your source just isn't that good. Most of the conversion programs work on the principle of 'garbage in, garbage out'. The cleaner the source is, the higher is the chance that the conversion result is nice looking.
Learning html/css is one way to make sure your source is clean. For epub creation it is probably even the best way.
If you don't have the time at the moment to do it or need a fast solution, cleaning up the Word file might work. Get yourself the
Smashwords Style Guide and rework your Word file according to their rules. The guide is even available in other languages if that helps you in any way.