The answer to this question may not be as straight-forward as we originally thought.
A research study was conducted by the Women's Health Initiative (WHI) to try and determine the benefits of hormone replacement therapy (HRT). In 2002, it was halted prematurely because the women participating in the study who were on both estrogen and progesterone had a statistically significant increase in breast cancer, blood clots, heart attack and strokes.
However, a more recent study (published in the January/February 2006 issue of The Journal of Women's Health) suggests that the increased risk may actually have been associated with the advanced age of the participants; most of the participants had been menopausal for a number of years before being started on any HRT. This new study also found that if women were started on HRT at the onset of menopause (and therefore at a younger age), then HRT appears to actually decrease the risk of heart disease.
A recent article in the Washington Post further explains this.