On win7 was set to 40h, had no troubles with tab completion in cmd.exe, never changed it..
I use powershell anyway..
PS C:\>cd 'HKLM:\SOFTWARE\Microsoft\Command Processor'
PS HKLM:\SOFTWARE\Microsoft\Command Processor>(gp .).CompletionChar
64
PS HKLM:\SOFTWARE\Microsoft\Command Processor>"0x{0:x}" -f (gp .).CompletionChar
0x40
PS HKLM:\SOFTWARE\Microsoft\Command Processor>sp . -name CompletionChar -value 9
PS HKLM:\SOFTWARE\Microsoft\Command Processor>(gp .).CompletionChar
9
PS HKLM:\SOFTWARE\Microsoft\Command Processor>sp . -name CompletionChar -value 64
PS HKLM:\SOFTWARE\Microsoft\Command Processor>"0x{0:x}" -f (gp .).CompletionChar
0x40
condensed:
PS C:\>sp 'HKLM:\SOFTWARE\Microsoft\Command Processor' -name CompletionChar -value 9