Hi there,
first things I would try is to start windows in safe mode (press f8 key rapidly when choose windows 7 to boot) if it works then run some antivirus software full scan.
second I would try Kaspersky Rescue Disk 10 google it its free and powerful tool it will boot linux from disk and scan your computer completely, also it will give access to your file on hdd if you will have to reinstall your system completely (hope not)
third shot is to insert Windows 7 installation disk and instead to click install now there is in bottom left corner link named Repair your computer after that click startup repair, also there is option to do system restore if you have created restore point earlier (if you do it often then you could try to do system restore FIRIST)
gl
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