About a "small"detail of the democracies

In Venezuela, before the presidential elections in December 2006, the most quoted opinion poll was an AP-Ipsos Poll that gave Chavez as a clear winner, but that also indicated in a slightly less circulated text that only 44% of the electorate believed that their votes would be counted accurately. Aside from whether the elections are factually clean or not, how much credibility should an election system have to demonstrate, a priori, before an election, in order to produce legitimate results?