You may have neglected to select the "Desktop" setup when installing. FYI, sarge is a few versions old, and so you'll have to fiddle around to get any packages for it.
You're probably better off installing this
version. It's the current stable version, Lenny 5.0, for older hardware. The default is an XFCE desktop system. If you would rather have LXDE as the desktop, type desktop=lxde
at the "boot:" prompt.
KDE or gnome are probably going to be too heavy to function nicely on this old laptop.