Read our forum sticky http://forums.cnet.com/5208-19411_102-0.html?threadID=344803.
The second (red) paragraph in the first post says "yes, you will keep most of your files and applications". But, of course, that's only guaranteed for the final release that will be available from October 22. Any earlier version might not be so advanced.
You might find the other information in the thread interesting also.
if i put win7 in can i keep all my files and program. or do i have to reinstall them.
i am using vista ultimate