My pc doesnt boot.... Plz HELP!!!!!

guys... i'm facing a weird
problem. i tried to install
win 7 (64bit) from a
bootable pendrive (as my
dvd writer is not working)
but the setup didnt
completely run and the
system was restored to
the previous OS (win 7 -
32bit). now whenever i turn my pc on after a few seconds it crashes and the os doesnt boot.
i cant reinstall windows 7
or xp. please give some
solutions to overcome
this problem
immediately. thanks in
advance.

please HELP!!!