file double in size when copied from virtualbox winxp to remote 9.10

I got a pretty weird problem. I have installed Win XP (32) using virtualbox (3.010) under ubuntu 9.04 (64 bits). And host home directory is mounted inside my guest winxp. I have another 9.10 (64 bits) machine and a folder is shared to my 9.04 machine through smbmount. The folder is mounted to ~/thinkpad in my 9.04 PC.

So inside my guest win xp, I have x: connected to my home folder in 9.04. and x:\thinkpad connected to the shared folder under 9.10.

The weird problem is that if I copy any file from win xp to x:\thinkpad (the shared folder under 9.10), the file is corrupted and essentially the size is double. I suspect it is an interoperability problem of 32 bit xp and 64 bit ubuntu. But I used to have the same setup and did not have any problem until I upgraded the remote PC from 9.04 to 9.10. Also the problem is only "one way". If I copy a file from the remote share folder (x:\thinkpad) into any local folder inside winxp, the file is not corrupted.