do you have the microsoft windows 7 basic installation disk?
put the disk in the drive and start the computer
if the disk will run, select: "upgrade"
this repairs the operating system in many cases.
this does not erase your personal files, but sets the
operating system back to a default state. hopefully
it runs again like normal.
I cannot say that it will work %100 of the time, but
if the computer is infected it might not work...
OR have a computer shop do any of the above for you.
Making a backup of all your personal files and pictures
(etc) is very important. After the computer is able to start
up again, make backups in case of future problems.