I had trouble getting that fix installed on one of my WinXP Pro systems (using WOU 7.4) and searching online forums told me I wasn't the only one. Unfortunately, none of the suggested solutions worked for me.
Finally, I resorted to Windows Update (WU) to see if that would make any difference and, surprisingly, it did!
So I decided to compare both downloads and, apparently, they were different.
WOU 7.4 yields the following download (for ENU):
"windowsxp-kb2686509-x86-enu_c7e2d5fc238a1566e5d7c5974e6ba5d3130c9c82.exe"
WU uses the following download:
"windowsxp-kb2686509-x86-enu_fdbb0ff8820e54be0eefc9e4e47ca4f0116d84a6.exe"
(coming from "http://au.download.windowsupdate.com/msdownload/update/software/secu/2012/04/")
Looking inside and comparing both packages (by executing them with a "/?" command parameter), the one from WOU 7.4 seems to be targeted at Windows XP Embedded.
Apart from a differently named DLL ("kblCheckerE.dll" vs. "kblChecker.dll"), two of the INF files ("update_SP3GDR.inf" and "update_SP3QFE.inf") in the WOU 7.4 download contain extra sections that point to WinXP Embedded:
[EmbNTPrereq.Section]
PresentOp = CheckReg, HKLM, "SYSTEM\CurrentControlSet\Control\ProductOptions", "ProductSuite", 0x00010000
EqualOp = CheckReg, HKLM, "SYSTEM\CurrentControlSet\Control\ProductOptions", "ProductSuite", 0x00010000, "==", "EmbeddedNT"
[KblPrerequisite.Section]
AndOp = EmbNTPrereq.Section
AndOp = Prereq.IsMachineSafe.Section
The NLD version (and probably other language versions too) of the KB2686509 fix from WOU 7.4 has the same problem.
FYI, I have another WinXP Pro system here where the KBKB2686509 fix installed without problem a couple of weeks ago using WOU 7.3.2...
Thanks in advance for looking into this issue...