Normally. when you do a new fresh install with Win7 ,it will save all your old data in a file(by default,if that partition have the system file in),named Windows old, in the same partition.Of course, if you intend to (re)install the OS,by doing the "PC format". You are not formating the PC, but the hard drive partition ...
Just be careful when you do the installation of Win 7 and read, i mean READ, the instructions and understand what you are doing. For safety,anyway, make a backup of your files in some other partition, like the other guys described in the other posts.