"I am in the windows feed back program which is only open to the first 500 applicants and I don't want to jeopardise
this "
The offer of the free Vista was a one-off. (That was the offer, by the way, not the feedback program). I cannot see that you will jeopardise anything as the offer is closed.
Downloading and installing any builds is unauthorised (not legal), and, as such, would, by its nature, be anonymous.
It is well posted on this site and others. Builds are not necessarily an advancement, as they are intended for other development sources and are leaked at random,and in no precise order.
If you love to explore Betas/builds, then go for it. Otherwise you will gain little. Better wait for an official release, which could be in the next few weeks..
The changes, at the moment, from one build to another, are not making any outstanding contributions. In fact (my own obsevation) 7057 is, in some respects, a small step back from 7048 (or 7000). It has some changed UAC settings, which Microsoft they would be attending to, as a result of negative feedback., there is a new logon screen, something which seems to change with every build,. There are a few new gui effects thrown in, such as wallpaper, and pictures for your personal account. It has, in the Eula, the remark "Microsoft Windows 7 Operating System Release Candidate 1". I would guess, that, as it is leading up fast to the RC, this will be normal in any builds from now, as it was in the final Vista final stages..