The current version of Oracle’s VirtualBox (4.0.8) has Version 4.0.8 had a regression bug which seems to only manifest itself when using Debian based hosts with 64-bit and Windows 7 64-bit guest system.
Here is the summary of the workaround posted as VirtualBox Ticket 8948:
Edit the settings file to allow the VM to start again (Replace control characters in line 315 with 4.0.8).
Start the Windows guest
HKLM\SOFTWARE\Oracle\VirtualBox Guest Additions
Create a string key
VersionEx(use exactly this spelling)
Give it a reasonable value (ie 4.0.8)
Shut down the VM.
Repeat step 1
Restart and shutdown windows guest again
Edit: This bug has been fixed in version 4.0.10 (released 2011-06-27).