IE9 crash during install and pc doesn't boot anymore!

I have win 7 Ultimate 64, with all the updates available installed,
including SP1. Genuine validated. Two languages installed: English and
Italian. In the past I’ve previously installed the IE9 RC and then,
after some days, I uninstalled it, and everything went fine. Today
I’ve downloaded the final version of IE9 (English language) 64 bit and
tried to install it. The program required me to close some open
programs which I did killing those tasks from the task manager, then
the installation completed asking me to restart (some files were in
use). I did but, during the subsequent boot the machine crashed and
reset, never coming to the logon mask.

I tried both the light install (around 500KB) and the complete install
( around 35MB) but nothing changed. After that I made a system repair
and then the machine rebooted just fine. The logs of the repair
process had no code errors (all the test reported 0x0 codes). So I
carried out the following checks :

· Chkdsk c: /f

· Sfc /scannow

· Scandisk and full AV scan

And I didn’t notice any error. I also tried to install it without
firewall or Av (ESET Smart Security v. 4.2.58.3) , but I got exactly
the same result and behavior. Any hint?

thanks in advance.

Luke