Microsoft has released Windows Vista Service Pack 1 via its Windows Update service, but will not start automatically sending it to users until "mid-April".
However, those who want SP1 now can go to the Microsoft website and download it.
From mid-April, users with Windows Update turned on will see Vista SP1 automatically downloaded, but it will not automatically be installed, said Microsoft.
SP1 has initially been released in five languages, including English, and more will follow later.
Microsoft recognises some application drivers may be affected when updating to SP1 and has published a list of known potential problems, with advice on how to deal with them.