Assuming that the hidden factory restore partition hasn't been deleted or written over, go to this archived Dell support article: https://web.archive.org/web/20171204032025/http://www.dell.com/support/article/us/en/19/sln291879/how-to-use-a-restore-point--factory-reset--or-manually-install-windows-7-on-your-dell-computer?lang=en
At the linked page, go to the section "Windows 7 Does Not Start Correctly or Not at All" and then select the tab "Using the Dell Factory Image Restore Partition" and follow the directions.