ABELLJMS,

Unfortunately, Windows 7 was never supported on this computer through Dell. There are no Windows 7 drivers available for download. Also, the computer never shipped with system restore. 

The OptiPlex™ 755 system could ship with one of five options for factory-loaded operating systems:

• Windows® XP Professional and Home Service Pack 2
• Windows Vista™ 32-bit Home Basic, Business, and Ultimate
• Windows Vista™ 64-bit Business 

If you have Windows 7 it isn't a Factory Image and likely a Retail License. You can DOwnload Windows 10 RS1 Media, make a Bootable USB set to use the MBR partition scheme for Legacy BIOS and clean install with your Windows 7 Retail Product Key:

http://dellwindowsreinstallationguide.com/download-windows-10-oem-and-retail-iso/ 

I'M  not sure this  is what your doing. Dell sold to me part# 342-5779 USB Recovery Key WIN 7 HOME PREMIUM 64 BIT CUI, KIT,WW.......But you are correct Dell never sold that unit with 7 I took the liberty to

stretchit's legs a fine machine. So give Dell a call and don't be surprised they no longer support Win 7. Go to E-Bay you,ll find it