try running
/usr/X11R6/bin/xvidtune
You can tweak the positioning of the desktop on the monitor using it.
Make sure you are logged in as root, and as the previous poster stated, there are various issues with SiS cards, and many of them need special options to be entered into the XF86Config file in order to work properly.
Just try maybe a search on google, or a seach on the xfree86.org site.
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