Which version of HW were you previously running?
Are you running 64 bit Windows 7? Current versions of HW need to have a 64 bit version of the Windows OS to run.
I would suggest applying all MS updates to Windows 7.
Try installing some other program (from a known good source) such as
http://www.winmd5.com/. If you can;t install this then you have a broken Windows system.
If this can be installed and run then make sure you have JAVA installed as the installer uses JAVA (HW doesn't). Try installing the latest Windows version of HW downloaded from the HW website.
Lookout for any messages from the install process.
Are you running on a user account on Windows 7 that has sufficient privilege to install software?
(If not try logging on as an administrator or try using "run as administrator" when you run the installer program.)
Good luck
Iain