Differences in google v/s Yahoo v/s MSN

My website ranking and presentation

[serengeti]

I have owned a site for close to four years. In yahoo and msn, it is ranked around 75-80 for the main keyword search. In google, it does not appear anywhere in the 500 results for the keyword search.

Also, on yahoo, its listed as xyz.com while in google, its listed as www.xyz.com (when I search for another secondary keyword to make the site appear on the results list).

Funnily enough, one of my pages has no external incoming links to it, but it appears in position 4 on google for a secondary keyword search. At the same time, that particular page appears nowhere on yahoo or msn for that secondary keyword search.

[serengeti]

Also, if one-third of my incoming links are because of my signature in a forum where I have made posts, does google penalize me? i.e., does the search engine think I am being deceiving?