I have succesfuly conected to my Slackware linux from my other computer with Windows xp on (I used Putty).
Now I wanna know, how can I start KDE (startx) over Putty, I tries with 'startx' but no luck.

Are you trying to boot kde remotely on your windows machine? If so you will need cygwin to pull that off.

Or are you wanting to boot kde locally on your linux machine? If so the first command you should try is "init 5"

I'm trying to boot it remotly.
So, cygwin is a windows based program for Kde?

cygwin is sort of a windows based linux, lol

Install it with all the X11 software. Then when all done run "xinit" that will start an x window with xterm.

Then run "ssh -Y user@ip_address" (The -Y is important as it enables X11 forwarding)

Then run "startkde"

If all goes as planned you should see the kde boot splash and then finally KDE.

EDIT: Perhaps there is a way to get KDE booted using putty. This is the way I do it and it works very well.

Basically what he is trying to say is that you need a X server on your Windows machine.
This can be either the one cygwin offers

or some commercial product.

I never heard that -Y is the option for X-forwarding.

The command you want to use from CLI is
if you plan on using it regularly and work over this connection
have a look at freeNX and the NXclient (client freely available at http://www.nomachine.org

iirc, puTTY is a text-only tunneler. No x-windows.

I've always had a bad experience with starting desktop environments over SSH X forwarding. Because all the desktop environment is forwarded to your local X server, it may conflict with the desktop environment or window manager already running locally. (Especially the time when I was using KDE in Linux and I SSH'ed to another computer and ran 'startkde'; didn't turn out so well ). Another way to get a remote "desktop" that is probably more efficient and flexible is VNC. Do you know about it?

For all the versions of OpenSSH beginning with 3.8 (that means all recent Linux systems within the last two years or so, including current packages in Cygwin), the -X option was changed to be untrusted forwarding, so that some X apps no longer work. The -Y option preserves the original (trusted) -X forwarding functionality. (see this).

No, Putty does has an option to forward X (Connection -> SSH -> X11 -> Enable X11 Forwarding), like all standard SSH clients; but you still need an X server for it to forward it to. The free X servers for Windows are mainly Cygwin/X, the X server in Cygwin, and Xming, essentially the same thing but without needing Cygwin.