I've wasted an hour at work trying various solutions posted here. Here's the latest log I got after an attempt at running WindowsXP-KB2686509-x86-ENU.exe:
[KB2686509.log]
0.579: ================================================================================
0.579: 2012/05/18 09:26:31.203 (local)
0.579: c:\b0c49dc4a077e7ad68\update\update.exe (version 6.3.13.0)
0.594: DoInstallation: GetProcAddress(InitializeCustomizationDLL) Returned: 0x7f
0.594: Hotfix started with following command line:
0.891: In Function GetReleaseSet, line 1240, RegQueryValueEx failed with error 0x2
0.891: CreateUserInterface: GetProcAddress for DefineInstallCustomUI failed: 0x7f
0.954: Return Value From IsMachineSafe = 0
0.954: IsMachineSafe returned 441092
0.954: Fist Condition in Prereq.IsMachineSafe.Section Failed
0.954: Condition Check for Line 1 of PreRequisite returned FALSE
0.954: ReadStringFromInf: UpdSpGetLineText failed: 0xe0000102
0.954: KB2686509 Setup encountered an error: Setup cannot continue because one or more prerequisites required to install KB2686509 failed. For More details check the Log File c:\windows\KB2686509.log
0.969: ReadStringFromInf: UpdSpGetLineText failed: 0xe0000102
0.969: Setup cannot continue because one or more prerequisites required to install KB2686509 failed. For More details check the Log File c:\windows\KB2686509.log
2.157: Message displayed to the user: Setup cannot continue because one or more prerequisites required to install KB2686509 failed. For More details check the Log File c:\windows\KB2686509.log
2.157: User Input: OK
2.157: Update.exe extended error code = 0xf0f4
2.157: Update.exe return code was masked to 0x643 for MSI custom action compliance.