Millions of business users face problems when installing Microsoft's big security update for Windows XP.
Home users have been downloading SP2
About 10% of all desktop machines will suffer compatibility problems following the installation of the SP2 update warns Canadian firm AssetMetrix.
In its survey the company found that small firms were more likely to suffer problems with their crop of PCs.
Also vulnerable were companies running older copies of applications or little-known versions of popular programs.
The long-awaited SP2 security update for Windows XP appeared in late August.
First promised in 2003, the SP2 update tries to make the XP operating system a less tempting target for virus writers, malicious hackers and tech-savvy criminals.
The software changes XP to make it easier for users to manage their anti-virus software, firewall and Windows updates.
SP2 also makes changes to the core of XP to close some of the coding loopholes exploited by many malicious programs and programmers.
CHANGES MADE BY SP2
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
But the roll out of the update has not been without its problems.
Microsoft itself has detailed 60 programs, many produced by Microsoft itself, that will not work as they used to once the SP2 update is installed.
In some cases tweaking Windows settings will solve the problems but in other cases more drastic action is needed.
Microsoft has let many corporate customers delay the installation of SP2 while they work out which programs it is likely to conflict with.
Now AssetMetrix has gathered data from large and small business users of Microsoft software to try to quantify the scale of problems that firms face.
On average it estimates that companies will have compatibility problems on 10.3% of their Windows PCs.
For smaller firms with less than 100 machines running XP this is likely to rise to 12%.
At the unluckiest firms in its survey, 60% of machines suffered compatibility problems once SP2 was installed.
Analysis of thousands of PCs by AssetMetrix showed that the older the version of software running on a desktop the more likely it was to suffer compatibility problems.
It also found that companies running versions of popular programs, such as spreadsheets, not produced by brand leaders could also cause problems.
AssetMetrix warns that Microsoft's amnesty on installing SP2 runs out on 16 December after which firms must install it.
"Our own testing and the feedback generated from customer testing indicate the overwhelming majority of applications work out of the box," said Paul Randle, Windows client product manager at Microsoft UK.
"The vast majority of application issues we have seen have simple workarounds, " he said, "very few fail completely."
He said Microsoft was working with business customers to let them set SP2 policies at a group level to make installation and post-installation tweaking easier.