Same here, autosandbox works on D drive but not on C drive. Both are on same disk.
Drive C → No autosandbox prompt, no sandboxing
Drive D → Autosandbox working normally
RAM drive (R:) → Autosandbox prompt appears, no sandboxing
USB thumb drive (F:) → Autosandbox working normally

Edit:
Huh, I found something strange…

2011/11/27 1:20:35	Autosandbox candidate: C:\autosandboxme.exe
	[Source: local://*C:\windows\explorer.exe		]
	[Opened by: C:\windows\explorer.exe]
	 --> Result: Not sandboxing (because the file is trusted).