Well, I was reinstalling Win7 for the first time in my life, so I was affraid of losing the large and important amount of data that I have on partition D. I installed Win7 in D (and then I was having 2 Win7 at the same time). Then I formatted drive C to get rid of the first Win7, and then I deleted the entry from System Configuration-Boot, so it remains just one. I hope you'll understand my bad english.