Microsoft is to start pushing installation of XP Service Pack 2 (SP2) over the next month and has released the first update to the update to fix a virtual private network (VPN) problem.

Microsoft has so far distributed the service pack to roughly 20 million users through Windows' Automatic Updates, many of whom are home and small business users. Soon, however, it will start pushing SP2's adoption to reach its goal of updating 100 million PCs by 18 October - two months after its release.

"Microsoft is aggressively monitoring and managing the download of SP2 and will be increasing the number of downloads over time," a spokesman said.

Microsoft has been gradually pushing SP2 and is still adding localised versions of the security-focused update. It is currently available in about 16 languages, to be expanded to 25 languages soon.

Shortly after SP2's release, a problem was found with VPN users. Microsoft made a temporary fix available and earlier this week released a permanent fix as an update, the first patch for the mammoth service pack.

The update, detailed in article 884020 in Microsoft's Knowledge Base, fixes a problem with applications attempting to connect to loop-back IP addresses other than 127.0.0.1. SP2 blocks all those addresses and users see an error message.

Microsoft has labelled SP2 a "critical" update and urges all Windows XP users to install it as soon as possible. As of March this year, Microsoft had sold about 210 million Windows XP licences, not counting volume licence sales to businesses. There are about 600 million Windows PCs in use worldwide.