Antivirus company AVG has had to issue roll-back instructions to users of the free edition of its AVG 2011 program after 64-bit versions of Windows fail to reboot correctly after a mandatory update.

With virus database 271.1.1/3292 applied on Windows 7, Vista and XP, an unknown number of users received the unhelpful message: ‘STOP: c0000135 The program can't start because %hs is missing from your computer. Try reinstalling the program to fix this problem.’

The company has now issued fix instructions on its website, ideally by using system restore in ‘safe mode’ to take the PC back to a point prior to the update. For users who don’t have this turned on, the company recommends using the AVG Rescue Disc to boot the system to manually remove the problem part of the program.

For users who don’t have that either, there is always the built-in Windows Startup Repair feature although judging from forum posts this might not work smoothly on Vista.

“This issue is limited to 64-bit systems only, currently it looks like not all 64-bit systems though, we are investigating. The updated has been pulled from our servers and if you have not been affected by this issue, it is OK to update normally,” commented an AVG engineer on the support forums.

The number of affected users is unknown but to does appear to be significant judging form the many pained messages on its user support forum.

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