How to activate GUI for Debian 3.1
 

I just install it on Microsoft Virtual PC 2004 but don't know how to switch from text mode to graphic mode.
Tell me how.

Did you install any graphical environments? Try typing "startx" and if it brings it up you're OK. If not try doing "apt-get install kdebase" to install KDE which is a nice desktop environment. You might also want to try Gnome, XFCE, Fluxbox, etc. and decide which graphical environment you like best.

Where I can download these desktop environments with fastest speed?

In the debian box type this:

apt-get install x-window-system

then

apt-get install xfce4
or
apt-get install kde-core

...