I formatted the hard disk and installed a new os. can recover the old files

[Chimhunga]

 

[tioynwyhwo]

it is a little difficult, but still u can have a try with a free data recovery software :
http://www.freenew.net/windows/icare-data-recovery-software-45/68220.htm launch it after installation and let it scan the hard disk, be patient, it may be a little long, then preview the files to decide whether save them. if the freeware can not do it, it means that the data disappear forever, of course u can also seek help from pros and let them decide.