I have the same problem installing USB_ANT_SiUSBXp_3_1.inf (as bundled with a Garmin 310XT) and SiUSBXp.inf (v 3.2, as downloaded from your website) on Windows Vista SP2. In both cases installation fails with "Error 5: Access is denied" in setupapi.dev.log:

flq: {FILE_QUEUE_COPY exit(0x00000000)}
flq: {_commit_file_queue}
flq: {SPFILENOTIFY_STARTQUEUE}
flq: {SPFILENOTIFY_STARTQUEUE - returned 0x00000001}
flq: {_commit_copy_subqueue}
flq: subqueue count=4
flq: {SPFILENOTIFY_STARTSUBQUEUE}
flq: {SPFILENOTIFY_STARTSUBQUEUE - returned 0x00000001}
flq: source media:
flq: SourcePath - [C:\Program Files\Garmin\USB_Drivers\ANT]
flq: SourceFile - [USB_ANT_SiUSBXp_3_1.inf]
flq: Flags - 0x00000000
flq: {_commit_copyfile}
flq: {SPFILENOTIFY_STARTCOPY}
flq: {SPFILENOTIFY_STARTCOPY - returned 0x00000001}
flq: CopyFile: 'C:\Program Files\Garmin\USB_Drivers\ANT\USB_ANT_SiUSBXp_3_1.inf'
flq: to: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\SETAD2.tmp'
flq: MoveFile: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\SETAD2.tmp'
flq: to: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\USB_ANT_SiUSBXp_3_1.inf'
flq: {SPFILENOTIFY_ENDCOPY}
flq: {SPFILENOTIFY_ENDCOPY - returned 0x00000000}
flq: {_commit_copyfile exit OK}
flq: {_commit_copyfile}
flq: {SPFILENOTIFY_STARTCOPY}
flq: {SPFILENOTIFY_STARTCOPY - returned 0x00000001}
flq: CopyFile: 'C:\Program Files\Garmin\USB_Drivers\ANT\DSI_SiUSBXp_3_1.cat'
flq: to: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\SETB31.tmp'
flq: MoveFile: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\SETB31.tmp'
flq: to: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\DSI_SiUSBXp_3_1.cat'
flq: {SPFILENOTIFY_ENDCOPY}
flq: {SPFILENOTIFY_ENDCOPY - returned 0x00000000}
flq: {_commit_copyfile exit OK}
flq: source media:
flq: Description - [Silicon Labs USBXpress Driver Installation Disc]
flq: SourcePath - [C:\Program Files\Garmin\USB_Drivers\ANT\x86]
flq: SourceFile - [dsi_siusbxp_3_1.sys]
flq: Flags - 0x00000000
flq: {_commit_copyfile}
flq: {SPFILENOTIFY_STARTCOPY}
flq: {SPFILENOTIFY_STARTCOPY - returned 0x00000001}
flq: CopyFile: 'C:\Program Files\Garmin\USB_Drivers\ANT\x86\dsi_siusbxp_3_1.sys'
flq: to: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\x86\SETBBE.tmp'
flq: MoveFile: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\x86\SETBBE.tmp'
flq: to: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\x86\dsi_siusbxp_3_1.sys'
flq: {SPFILENOTIFY_ENDCOPY}
flq: {SPFILENOTIFY_ENDCOPY - returned 0x00000000}
flq: {_commit_copyfile exit OK}
flq: {_commit_copyfile}
flq: {SPFILENOTIFY_STARTCOPY}
flq: {SPFILENOTIFY_STARTCOPY - returned 0x00000001}
flq: CopyFile: 'C:\Program Files\Garmin\USB_Drivers\ANT\x86\silib.sys'
flq: to: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\x86\SETBFD.tmp'
flq: MoveFile: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\x86\SETBFD.tmp'
flq: to: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\x86\silib.sys'
flq: {SPFILENOTIFY_ENDCOPY}
flq: {SPFILENOTIFY_ENDCOPY - returned 0x00000000}
flq: {_commit_copyfile exit OK}
flq: {SPFILENOTIFY_ENDSUBQUEUE}
flq: {SPFILENOTIFY_ENDSUBQUEUE - returned 0x00000000}
flq: {_commit_copy_subqueue exit OK}
flq: {SPFILENOTIFY_ENDQUEUE}
flq: {SPFILENOTIFY_ENDQUEUE - returned 0x00000000}
flq: {_commit_file_queue exit OK}
sto: Driver Package successfully copied to C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}
inf: Opened INF: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\USB_ANT_SiUSBXp_3_1.inf' ([strings] )
pol: {Driver package policy check} 21:49:05.572
pol: {Driver package policy check - exit(0x00000000)} 21:49:05.573
dvi: Staging Package To Driver Store - phase 2
! inf: Unable to load INF: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\USB_ANT_SiUSBXp_3_1.inf'(00000005)
! inf: Error 5: Access is denied.
! inf: Unable to load INF: 'C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\USB_ANT_SiUSBXp_3_1.inf'(00000005)
! inf: Error 5: Access is denied.
!!! sto: Failed to copy driver package from user'ss location to secure location.
!!! sto: Error = 80070005
!!! sto: SrcInf = C:\Users\adrian\{5cd75ba8-6d2f-4c7e-8a6f-1c6baf454a4b}\USB_ANT_SiUSBXp_3_1.inf
!!! sto: DestPath = C:\Windows\system32\DriverStore\Temp\{4e8bce21-c678-4546-9728-6fdf470974c1}\Package
sto: {Staging Package To Driver Store - phase 2 exit(80070005)}
sto: {Staging Package To Driver Store - phase 1 exit(80070005)}
! inf: Add to Driver Store unsuccessful
! inf: Error 5: Access is denied.
!!! inf: returning failure to SetupCopyOEMInf
<<< Section end 2010/09/03 21:49:05.739
<<< [Exit status: FAILURE(0x00000005)]

I've tried installing in safe mode, as Administrator, and I've tried decrypting my home directory from which I launched the installer, all to no avail. The 3.1 version installed successfully on a different Vista computer.

I would be most grateful if anyone could help diagnose and overcome this problem.

Thanks & regards,

After much experimentation I finally got the original Garmin-bundled driver to install. I had to:
- Uninstall all Garmin Ant Agent + USB driver software (other than GarminConnect)
- Delete all Garmin Ant Agent-related registry entries (again, other than GC)
- Delete C:\Program Files\Garmin directory entirely
- Run the Ant Agent installation as Administrator

Thanks,