Symantec has warned customers that installing Windows XP Service Pack 3 could harm their computers unless they switch off a feature of their security software first.

Reese Anschultz, a senior manager at Symantec, said in a post on the security firm's support forum that consumers using Norton Internet Security, Norton AntiVirus and Norton 360 must switch off the 'SymProtect' feature before trying to install XP SP3 or would find additional registry keys installed with the existing keys.

"After a lot of testing, we've reproduced a number of different cases where applying the XP SP3 upgrade adds additional registry keys within already-existing Symantec registry keys," said Anschultz.

"We have determined that the SymProtect feature is involved, though this issue is not exclusive to Symantec customers. To help prevent this issue from occurring, you should disable SymProtect prior to installing the Windows XP SP3 upgrade."

SymProtect, which Symantec bills as technology designed to protect its security software from being hacked by malware, is tagged with the added description of 'tamper protection' in some of its consumer products.

Anschultz said Symantec was still trying to pin down why the XP SP3 upgrade was adding spurious entries, in some cases thousands of them, to some users' Windows registries. "We are still trying to understand why the upgrade is adding these keys," he said in the post.

Within hours of Microsoft releasing Windows XP SP3 to its Windows Update service, users started reporting problems that included empty Device Managers and broken network connections that cut them off from the internet or wireless networks. Users quickly identified the cause as large numbers of bogus keys, all starting with nonsense characters such as " "$%&", that had flooded the registry.

Symantec has repeatedly denied the problem was its fault, while Microsoft has just as regularly declined to comment. Last week, Symantec blamed a Microsoft file named fixccs.exe for spawning the errant registry entries; fixccs.exe is part of the XP SP3 upgrade package.

However, a Symantec spokeswoman has since said that it was a combination of fixccs.exe and her firm's SymProtect that caused the problem on some systems. "Fixccs.exe adds registry keys during the SP3 update process and then attempts to delete them," said Cecilia Daclan.

"SymProtect prevents changes to the registry keys. Thus, it prevents the deletion of the keys added by fixccs.exe."

Symantec continued to claim, however, that the blanked Device Manager problem was not specific to its software. "This issue is not exclusive to Symantec customers," said Daclan. "We've seen reports from various users who are not running Symantec products."

Last week, Symantec said it was working on a stand-alone tool that would delete the extraneous registry entries, and added that it hoped to have something "pretty quickly", although it has yet to be released. Anschultz said the company would use the support forum to point customers to the tool when it was finished.