Windows 7 Explorer Failures - Unsolvable for the last 17 months now.
This one's been around for as long as Windows 7 has been (all variants and versions), The symptoms vary between long-lasting hour glasses and an Explorer screen vanish (being replaced by a pop-up saying "Windows Explorer has failed, searching for a solution to this problem"). After a while it fades away and you can try to re-open Windows Explorer again, only to be subjected quite soonish to the same indignity.
It has (apparently) never been resolved by Microsoft and has all the characteristics of a vintage Win 98 stack overflow. If you GOOGLE the malady, you'll find literally hundreds of thousands of posts on the problem. They typically have claims of having found a solution, only to later post that they were mistaken - i.e. like Arnie Schwarz, "it's back".
I've got it on a server, a laptop and a desktop and it's really annoying. People talk about Vista's problems being a disincentive, however this beats the Vista vagaries hands down.
So if you haven't yet upgraded to Windows 7, I suggest you don't - if you value your sanity.