Microsoft's big security update for Windows XP is getting mixed reviews from the first people to install it.
XP was launched in late 2001
While many people have said SP2 caused no problems, others say it can conflict with some non-Microsoft programs.
One online forum collecting SP2 stories shows a majority of respondents reporting problems ranging from minor to severe.
Microsoft is letting some customers postpone installation until they are sure it will not crash other programs.
"Our own testing and the feedback generated from customer testing indicate the overwhelming majority of applications will work out of the box," said a Microsoft spokesperson.
"Some of the capabilities in SP2 may change existing application behaviour and the customer experience with those applications, particularly custom-built web applications in large organisations.
"In such cases, we've been working with developers and IT administrators throughout the SP2 testing process to make necessary updates in order to ease implementation when the final release version of SP2 is made available broadly."
Business users have been the first to get their hands on SP2 which Microsoft first promised in 2003.
Microsoft has released tools that let firms stop the update installing across business networks.
The long-awaited update is intended to make Windows XP a much less tempting target for virus writers and malicious hackers.
SP2 KNOWN ISSUES
Conflicts with LaCie Firewire 800 drives
Causes problems with SQL Server 2000 and Microsoft CRM 1.2
Could break net tools that use raw sockets
Microsoft Systems Management Server Remote Tools cannot remotely manage clients running SP2
SP2 firewall disables Client Push Installation
SP2 users must update to Microsoft Baseline Security Analyzer 1.2.1
The most visible change is the Security Center which gives users one place to manage anti-virus software, firewall and updates to the operating system.
Under the hood SP2 makes big changes to the way that PCs handle programs that in the past have been used to smuggle malicious software on to a machine.
But some Microsoft customers are concerned about the scale of the changes.
IBM reportedly placed a notice on its intranet telling users not to install SP2 until it has produced a customised version that will not conflict with the applications it runs internally.
Microsoft has also allowed corporate customers to suspend the updating system that SP2 uses to automatically install itself.
Pop-up ads blocked
Revamped firewall on by default
Outlook Express, Internet Explorer and Windows Messenger warn about attachments
Origins of downloaded files logged
Web graphics in e-mail no longer loaded by default
Some spyware blocked
Users regularly reminded about Windows Updates
Security Center brings together information about anti-virus, updates and firewall
Protection against buffer over-runs
Windows Messenger Service turned off by default
The respected Sans Institute has set up a forum on its website to collect SP2 installation stories.
At the time of writing it had reports from 152 users about their experiences.
Although 43% said the SP2 installation had gone without a hitch, 49% of those contributing had problems ranging from minor to severe.
A few contributors said they had to completely rebuild a system before they could get the update to work.
Many others, 30% of those responding, said they had minor problems such as clashes with non-Microsoft browsers or applications.
Some had to tweak hardware settings to get machines running as they did before SP2 was installed.
External drive maker LaCie has produced a firmware update for its Firewire 800 products to make them work with SP2.
But the overall reports about SP2 were broadly positive.
Some of those responding mentioned that it took a couple of hours to install the update which, for business firms, weighs in at 266MB.
The version for home users is likely to vary in size depending on whether users have kept up to date with past updates to XP.
Before installing SP2, users are recommended to ensure their machine is free of viruses and Trojans that could put it under the control of malicious hackers.
Although SP2 will make machines less vulnerable to many common viruses it will not spot if machines have been subverted to become spam-spewing relays.