The windows installer for IDEA 10 is still broken as it was in the previous versions:
"Please select an empty folder or a folder where IntelliJ IDEA Community Edition 9.0 is installed as the target for installing IntelliJ IDEA Community Edition."
which is exactly what I did:
C:\Program Files (x86)\JetBrains\IntelliJ IDEA (contains 9.0.4 Community Edition installation)
Windows 7, 64
Here are some of the older threads with the same problem: