Normally you just need to make sure that you have .NET installed and then Windows Update will lead you through all of the update. However I've run into failures like yours on about 1 in 50 servers, so the trick is to figure out the best path to navigate for your situation.
You're on the right path. Here's what I suggest:
For the WS08 server, I would uninstall the .NET Framework completely and then build back up again. To do so, use Add/Remote programs to uninstall any of the service packs that you see there. Then from Server Manager go to -> Roles -> Web Server (IIS) -> Remove Role Services -> and remove "ASP.NET" and ".NET Extensibility" (and accept the prompt to remove dependencies). After that completes, perform a reboot if it asks for one.
Then, from Server Manager again, add a role service and add those back again. Then use Windows Update, but step through the service packs in order. Don't assume that the cumulative installs will work since you already know that you're a fringe case.
For the Vista computer, if you haven't already, I suggest the Windows Update path there too. If that dead-ends, then start here: http://www.microsoft.com/net/Download.aspx. Make sure that you do have .NET 2.0 installed. (just check c:\windows\microsoft.net\framework\v2.0.50727 and make sure it has the full set of files.).
Then install 3.5 (not SP1 yet): http://www.microsoft.com/downloads/details.aspx?FamilyId=333325FD-AE52-4E35-B531-508D977D32A6&displaylang=en
Finally 3.5 SP1: http://www.microsoft.com/downloads/details.aspx?FamilyId=AB99342F-5D1A-413D-8319-81DA479AB0D7&displaylang=en
Hope that helps.
Best Answer
Windows Update does that quite nicely. No need to play around with non standard software. Why not use that out of the box functionality?