A new test build called Windows Vista SP1 RC preview was released to 15,000 private beta testers this week.
The test build incorporates setup and installation changes based on feedback from current SP1 beta testers, but few other details were provided by Microsoft about the build.
SP1 RC will be available to a larger group of testers "soon," said Microsoft.
The first test build, Windows Vista SP1 beta, went out to about 10,000 testers in September of this year.
Microsoft has steadily been releasing fixes for Vista through Windows Update, such as a fix released this week for a USB-related problem that causes Vista to crash.