I've spent 24 hours getting to a successful update of 3.5 SP 1 & Fam Update (shows as single update under Control Panel --> Windows Update. Just wanted to share my experience in the hope it might help others.

I'm running Vista Ultimate x64 on a Dell Dimension 9100 (dual core 2.8GHz). My Internet connection is through a Sprint/Sierra Wireless USB 598 EVDO Network Adapter -- and I'm out in the country and ave. app. 0.15Mb/sec download speeds. Yesterday, was running with .5GB main memory. As most readers will know, this was a huge mistake. (Was waiting impatiently for 4GB memory upgrade to arrive.) The 3.5 SP 1 & Fam Update is required by some third party applications -- and, of course, by anyone interested in doing Viz Studio development of new software. This update would simply not install. Install failed, re-install failed, tried Reg Cleaner and still failed. Got to point where Restart was in an infinite loop.

Installed 4GB main memory. Did a complete re-install of Vista Ultimate x64. Then set the Windows Update running - there were 51 updates selected - 49 Important or Recommended Updates and two Optional updates I selected. At the end of this process there was one glaring failure - our friend .NET Frmwrk 3.5 SP 1 & Fam Update. I spotted one post suggesting that simply retrying might solve update failure problems (generally). There was one additional update available - (KB943729), "Group Policy Preference ... " that I checked for installation. I selected both updates - and it worked.

Acknowleding they are based on one person's experience, here are my suggestions for fellow sufferers of hassles with the .NET Frmwrk 3.5 SP1 & Fam Update:

• Vista Ultimate x64 - don't even think of running it on .5GB main memory. Of course, I don't know the "break point" - but am confident it is way closer to 4GB than to .5GB.
• Connectivity - I cannot rule out that my slower download speeds were a factor (although I also cannot explain why corrupted or dropped packets would not be detected and resent).
• Avoid Suggested Hacks - There are numerous suggestions out on the web for fixing this particular failure to update. DON'T TRY THEM. The update does - eventually, and after much distress - work. Even for the true geeks, these suggestions caused me to make a bad situation worse.
• Windows Update First - My successful Windows Update of the .NET Frmwrk 3.5 SP 1 & Fam Update occured after my clean re-install of Vista Ultimate x64, installing the Sprint S/W for my network adapter (and absolutely nothing else), not running any other software on the system and running Windows Update.
• Try Again - The first time around, the update failed. I re-tried the update as I was going to post a "help me, please" note to this forum. Noticed that the re-try was successful.

So memory, patience and persistence. Enjoy.

Thanks for reporting your experience.
Safe surfing.