FYI- we played MS's fairly easy (read lame) games for a weekend and received enough points for a free copy of Vista Ultimate. That was when you only had to accumulate 6,000 points. The last few weeks the point threshold has risen to 20,000 for Home Premium.
Each game gives different points, I think Flexicon (simple crosswords) gives the most. A couple of months ago they had Ultimate Office for 6,000 too.
Some prizes are bargains, some not.