Google does not manually assign keywords to your site, nor do Google manually “boost” the rankings of any site. The ranking process is completely automated and depends on the relative PageRank of each result found.
The best way to improve your position in results is to have relevant content and multiple links from other web sites. If there are certain keywords you feel are essential to your site’s success, you may want to consider a targeted keyword advertising program.
Google does not sell placement in the results, but Google do have advertising positions available adjacent to them.
Excerpt taken from Google Webmaster Info