Security update for the .NET Framework 1.1 SP1 for Windows XP, Windows Vista and Windows Server 2008, and for x64 and Itanium-based versions of Windows Server 2003: October 11, 2011 ---- This is the one that will not download.

If your sure you actually have a 64 bit processor and OS you can try downloading it manually and installing that way. Sometimes autodownloads fail because an AV program is falsely seeing the download as a threat. If you search the KB number on the MS support site and download it manually and it still fails to install, try disconnecting from the intenet, turn off your antivirus and fire wall and install manually. Then be sure to start the securitys or reboot the PC before reconnecting to the internet. Firewalls will sometimes falsly target a download too and that is when its really necessary to disconect from the net not necessarily when just turning off the AV so I would try that first.

OH, also if you go into add remove programs (in XP at least) and check the box for "show updates" you will see what .nets you have installed. You can not install the sp1 until the original is installed and functional. There is also a chance the original is corrupted in some way and is not being properly recognized so reinstalling it may also solve your problem.

TBH, as Micro$oft are now on .NET 4 you do not need this update anyway, so best to cancel it.
Also unauthorise it installation.
That'll resolve your issue.

Now it's time to head over to the MS updates site and make sure you have all that you actually need, rather than the automated bear minimum..

Also another suggestion when having trouble with the automatic updates is to do it the manual way but when you go to start updates choose "run as administrator". Some core files won't update without that permission first.