I have ati radeon hd2400, I downloaded drivers from ati's site and install it on mu laptop. How to install compiz on Slackware 13.1 (x64) ?

you do not need them drivers should work ok with the stock ati driver you will find the one in the kernel is more up to date.If I am wrong please let me know but that is what the web site says. That said.
I use sbopkg here download then I install it and then type su - enter then password enter type sbopkg then search compiz in the sbopkg browser you will find you have to install yasm it says that. and it must be installed in order.
you must read the post install info or you will have no borders.read the read me.
other wise go here and do it the long way.
and while your at it put your own firefox slackbuild searh plugin in your browser here
now your cooking.

The fullset of compiz packages are available for 13.1 over at slackbuilds.org. Just make sure you read the readme files before you install.

Does this work with XFCE ?

Yes, works perfectly. The only issue I have with compiz is I had to recompile the stock compiz package that ships with slackware because it was crashing with KDE4-Window-Decorator. After I recompiled it worked fine, but this should not affect Xfce.

yes, work like a charm !!!