This has just started happening in the last couple of days. Acer PC running Windows 7 64-bit, no recent hardware or software changes. I don't have a Windows 7 installation disk.
The computer starts booting up normally, goes through the Windows initialization screen and the Acer welcome screen, but then instead of loading my desktop it goes to an empty black screen with only the cursor in it. The cursor moves but there is nothing to click on.
Rebooting the machine by holding down the power button has so far eventually resulted in the desktop coming up, and the computer runs normally from there. On one occasion it required two consecutive reboots.
I ran a chkdsk which found nothing wrong. I also looked at the registry and saw under WINLOGON that Shell is set to explorer.exe. Since the system has (so far) eventually loaded by rebooting with no changes, I have to assume this is a problem with execution rather than configuration.
I am now busy making sure all my data is backed up, but am afraid to shut off my machine again until I figure out what's going on.