I actually developed my own computer rankings two years ago...this is the third season I've been running them. I'll spare this forum the boredom of the details of the math (PM if that's your sort of thing), but in summary it is driven by who you've played and how well you played against them. So strength of schedule is a biggie, as is margin of victory (to some degree), home/away, and opponents' schedules. I have it set up in Excel--all I do is enter the scores and a macro iteratively calculates the results. There are no preseason rankings in my system--everyone starts the season the same.
I can supply the Top 25 from my program, although it takes a few weeks for the teams to interconnect so there are a couple of head-scratchers these first couple of weeks.