Will Microsoft finish Windows Vista on schedule? That's one of the biggest questions facing the company these days. And the completion of the first Windows Vista release candidate last week creates a new opportunity to assess its chances, using historical comparisons to previous Windows timetables.

Click for large chart. Sources of historical dates: ActiveWin; and Microsoft 1, 2, 3, 4, 5.

See this chart for timelines depicting the final stages of Windows 2000 and Windows XP development -- showing the number of days from Release Candidate 1 (RC1) to Release to Manufacturing (RTM) and then product launch. Below those, for comparison, is a third bar showing a hypothetical Windows Vista timeline, starting with last week's RC1. That timeline reflects Vista's scheduled November business release and January retail release, giving Microsoft maximum wiggle room by pegging the dates to the end of each month. It also assumes arbitrarily, for sake of comparison, that Microsoft would need to release Vista to manufacturing around the end of October to meet that schedule.

The chart shows that Microsoft was actually able to move Windows XP from RC1 to launch considerably faster than Windows Vista's retail release schedule would require. Based on that, it might be tempting to think that it will be easy for Microsoft to make the Vista release schedule, with time to spare.

However, there are some key differences with Windows Vista. For one, even though Vista's scheduled business launch stands to be less logistically complicated for Microsoft than a full retail rollout, a November launch would come no more than 90 days after the RC1 completion. That's a quick turnaround, compared with the timetables for the Windows XP and 2000 releases.

You could also argue, based on the amount of time and the magnitude of changes between releases, that it would be more appropriate to compare Windows Vista with Windows 2000. (In other words, the company's big upgrade from Windows XP to Windows Vista would be akin to the jump from Windows NT 4.0 to Windows 2000.) And while Windows 2000 was a unique situation, it's worth noting that the amount of time between its RC1 and release was especially long.

And there's another big difference this time around: At this point in the process of Windows XP's development, announcing the completion of XP RC1, Microsoft was precise about the final release date, saying publicly that it would be 115 days later, on Oct. 25, 2001. Windows XP ultimately met that schedule.

In contrast, the company's public statements about the Windows Vista schedule haven't been nearly as precise. Despite completing Windows Vista RC1 last week, Microsoft hasn't yet announced specific days in either November or January for Vista's business and retail releases. Of course, time will tell whether Windows Vista makes the current timetable. But in addition to the previous equivocation from the company's top executives, and the ongoing doubts expressed by analysts, the absence of specific public dates at this point indicates a considerably lower degree of confidence in the schedule this time around.