In my CentOS 6.3 computer, I installed a MS Server 2008 through a VirtualBox. Is it possible to monitor the network traffic/activities of my MS Server 2008 in my CentOS? How can I do that?
You should investigate how to use SNMP monitoring with Windows Server.
SNMP is a protocol that allows standard reporting "traps" to be sent to a monitoring machine. You need something then to monitor the messages on the Linux host such as Nagios with perhaps a suitable GUI, see the site linked for a list.
An alternative tool though probably rather overkill for your use is OpenNMS. I am sure that there are plenty of other SNMP tools that you can also run on Linux.
UPDATE: From the comment, I obviously got this the wrong way around. My personal preference for monitoring and administering Linux systems "remotely" is to use Webmin. This installs a stand-alone web interface on port 10000 by default and works really well. A little bit OTT for what you want but simple to install, well supported on most Linux versions and pretty secure.
You can, of course, use SNMP monitoring as mentioned above just the other way around, you would want an SNMP monitor/viewer for Windows of which there are a few but this is more complex to set up.