You should have a Windows 7 disc. Insert it into the computer, power down and hold F12 while booting up, select boot from CD/DVD, follow the onscreen instructions until you have installed Windows 7. You will need to install Windows 7 Service Pack 1 (best to use the standalone version) and then install the Windows 7 drivers (I guess you meant Optiplex 380). The Dell Drivers are available here and the standalone service pack is here. You will need to check if you are installing 32 (x86) or 64 (x64) bit windows.
I bought my Potiflex 380 descktop that came with Windows 7, but I had it rolled back to XP. I now want to upgrade to 7. Do I use the reinstallation DVD for this or do I do something different--I looked and can't tell if Windows 7 is installed.
Windows 7