Anyways, the Windows 7 looks great although it's not an official release version.
See it on my computer:
There are some small issues after installing:
1. It can not recognize the Shared folders of Virtual Box automatically like Windows XP. I have to share the folder in the host machine and use network drive mapping in Windows 7 to share data.
2. It can not recognize my USB: currently, I don't know why. However, I am using the solution for issue 1 to solve this also.
Update 1:
I found a new solution to solve issue 1: actually, Windows7 does not show shared folders in \\vboxsvr but it can recognize them. You just need to open the Console to run "net use" command and map it with a specific drive.
Example:
net use T: \\vboxsvr\SharedFolderInVirtualBox
T is the drive letter that you want to map.
