It sounds like to me an infection did this to your pc.
I would try to first use a system restore point to before this occurred. Use option 2 in this guide since you cannot boot into windows that well:
System Restore
If this fails,
Try a Repair Install. This way you will not lose your data.
Once again, another important reason to have your data backed up:
Backup Complete Computer - Create an Image Backup
Backup User and System Files
^^^ agreed, you should be able to boot from your install media either your original install disk or a bootable usb flash you can create
this will enable you to boot into windows recovery environment which will give you the options above, i would try system restore first followed by a repair install and then only if all else fails a re-install
Just incase you dont have a win7 install dvd you can download the appropriate win 7 iso from here Windows 7 Direct Download Links
and use this tool to create a bootable installer on a flash drive Windows 7 USB/DVD Download Tool
note* when downloading iso files be sure you select the correct version of windows to match your licence key :)
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