BE20 pilot reports descending below minimums while attempting to circle for landing at minimums off the VOR D approach to IWS. A missed approach ensued and the next approach using a straight in GPS approach was successful.

Narrative

How problem arose: On the VOR D Approach to IWS; as I made my turn to circle I may have descended below MDA. I turned up downwind lost sight of the runway and executed a missed approach. I requested the GPS Approach to IWS; landing was uneventful. Low IFR IMC ceiling's were ragged. Eyes outside the cockpit when I had a visual on the runway. How it was discovered: As I made my turn to circle I glanced at my altimeter I was under 500 FT MSL and made the corrections. Climbed to MDA and executed missed approach. Human Performance Considerations: Weather reports caused me to anticipate low visibility and a possible missed approach. Hindsight I should have planned to land at a different airport with an ILS. Actions/In actions: By not keeping up a better scan I may have descended below minimum. Keeping my IMC scan quicker will assist in preventing future episodes. Factors: The weather was forecast to improve prior to my arrival at IWS. The weather did not improve upon arrival. In the future; I will make greater efforts; in the interest of safety; to keep my cockpit scan up to speed. Plan to use airports that have Precision approaches when weather conditions are unfavorable.