Hi daniel joy500, when you format a hdd, you destroy all data on it. What you are asking is how to install windows 7 without losing all your data from previous OS.
What are your pc spec, CPU, MOTHERBOARD, RAM, GPU, type of hdd ( sata or ide ) and the previous operating system you are using.
Never upgraded myself, always do a clean install, but upgrade may save user data, don't quote me on that :-)
To fresh install, you WOULD HAVE TO BACK UP ALL YOUR DATA to another hdd, or USB memory stick ( not practical really if you have 100's of Gb of data ) or external hdd or DVD.
Update thread daniel joy500 with the information needed above, then we can better advise you on the process to follow so all your important data is not lost when you upgrade to win 7.