I had a bit of trouble installing .NET 3.5 recently. My machine had decided to install some Windows Updates at the same time as installing .NET 3.5 and I think the combination of the two caused the entire machine to fall over in a heap. One by one all my applications stopped responding, including ctrl+alt+del, so I resorted to hitting the Big Reset Button, but this left my 3.5 install in a very broken state. It wasn't installed and it couldn't be installed - each time I tried, Setup threw an exception.
I looked through all the installed programs, trawled through all the logs, but couldn't see why I couldn't install 3.5. Then I came across Aaron Stebner's blog, where he lists a load of tools and articles that could help - after trying a couple of other tools, I struck gold when I clicked the the What to do if other .NET Framework setup troubleshooting steps do not help link! Finally, I was able to install .NET 3.5, and all is well with the world once more.