Thank you reinfo and PeteC.
I am very much concerned about the Windows 7 security now.
1: If the native/third-party firewall, anti-virus software and Internet Security software running in Windows 7 could not cover Windows XP Mode and Windows Virtual PC, then a PC could be infected at any time whenever Windows XP Mode/Virtual PC is running, even though Windows 7 environment is well-protected. If this deduction is true, that means Windows XP Mode and Windows Virtual PC can be un-locked backdoors of Windows 7 security. Am I correct?
2: Avast or Avira. PeteC, how's your experience of their services (assuming the product is as good as Norton 306/Internet Security), in case the installatation got into trouble.
3. If I use Avast or Avira, do I need / stop using the Windows Defender and/or Windows Firewall in Windows 7. Do I need a Windows 7 (64-bit) registry clearner? What's your recommendation? Anything else that I need?