I installed the pidgin, set up my account and password of msn, started pidgin,
it appeared "Available - waiting for network connection", but it just kept
waiting , not logged in. At the same time, Kopete worked perfectly on the exact
same pc.

My pc uses wireless network card (usb interface) ,  and it was brought up in
the /etc/network/interfaces. After reporting this bug, I tried comment out
the setting in /etc/network/interfaces like below:

# The primary network interface
# allow-hotplug wlan0
# iface wlan0 inet dhcp
        # wireless-* options are implemented by the wireless-tools package
#       wireless-mode managed
#       wireless-essid mynetwork
#       wireless-key1 s:mypasswd

and began to use gnome network manager, strangely the pidgin can connect to
the msn now.
I don't know why.