Microsoft has made the first service pack (SP1) for the Windows Vista operating system available for download from its web site.

The upgrade was released to systems manufacturers selling new desktop and laptop PCs last month, but this is the first time SP1 has been available to people using Vista on their existing systems.

Users can choose to either download the 450MB file update (727MB for 64-bit Vista users) manually now, or wait until mid April when the automatic update option in Windows Update will start downloading SP1 to PCs automatically if it has been configured to do so, according to a blog post by Windows Vista product manager Nick White.

The number of people who have already migrated to Vista from previous versions of Microsoft operating systems is unknown. SP1's arrival is likely to provide many with the reassurance they have been waiting for that any initial problems with the software, first released in January 2007, have now been addressed.

The update claims to include a range of performance, reliability, security and power consumption refinements as well as new hardware drivers and application compatibility tweaks.