Cannot factory reset my Optiplex 990

Hello,

I am writing this as I have tried multiple ways to factory reset my Optiplex 990, to no avail. I've tried the Advanced Boot Options menu, and there was no factory image, and when I tried using the Dell OS Recovery Tool, it told me my system is no longer supported.

This is a second hand computer so I'm not sure what other options i have. I simply want to give this computer a fresh start, and I absolutely want to keep Windows 7.

Thank you for your assistance.