Can't boot into windows 7. Goes into System diagnostics instead of windows?

when you boot in your older version of windows if it is running still then you see the properties of the drive and find the amount of gbs you decided to keep in that patition and see the another patition where you installed w7, now if your older window is running perfectly then install w7 clean by booting it from the dvd, but you must need a bootable dvd to install otherwise you will need to open setup from the olderversion of windows(like xp) and when you install this w7 newly select advanced option and format the partition where you r going to install w7 so first record the bytes allocated for the drive partition. get expart help if you r not experianced in this type of works