Usually when LimeWire eats up large amounts of CPU it is either a Java problem or a Graphics problem the 3rd link birdy gave you should take care of any java problem but make sure you uninstal all previous versions of java when you upgrade java it does not un install previous versions you can go to add and remove programs to do that.
The link below will take you to the known graphics problem there are a couple of other users having the same problem as you with the 7800.
http://www.gnutellaforums.com/showth...=&pagenumber=2