After installing the Microsoft security update 955759, the users who have installed Windows XP and the Citrix Access Gateway plug-in on the computer observe that the explorer.exe process crashes.

This issue appears to be caused by changes in the operating system made by Microsoft in the update 955759. The issue is limited to Virtual Private Network (VPN) connections initiated by using the Internet Explorer Web browser. The VPN connections started directly from the computer or by using an alternate Web browser are not affected.

Alternatively, the following workarounds also exist:

• Disable the AppCompat feature of the Iexplore.exe process by completing the procedure available in the Microsoft Support article.

• Start the VPN tunnel from the Citrix Access Gateway plug-in or from an alternate Web browser, such as Mozilla Firefox or Google Chrome.

• Uninstall the Microsoft Update 955759.

My workaround:

Create a registry file with following content:

APPCOMPAT-Iexplorer-fix.reg

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags]
"{5042648C-F439-468D-859B-6CD12BA02D3A}"=dword:00000001

Apply that registry patch to the client.

 originally found at support.citrix.com and forums.citrix.com