Dell driver optiplex gx620

Dell do not supply Win7 drivers for this model. However, Win 7 will probably have enough drivers to run the system, probable exceptions are Chipset and Video. You could try the XP drivers from
http://www.dell.com/support/drivers/us/en/19
Once the driver setup file is downloaded, Rt click on the exe file and choose 'Troubleshoot Compatibility' and it will probably run OK. Worth a try anyway!