Dell do not make any Windows 7 drivers available for the OptiPlex GX280 even though this machine can run it acceptably (32bit only). Where I work there are a few of these machines which will be kept as spares if any of the main stock of OptiPlex GX620 machines fail and need repairing, and for miscellaneous duties like driving plasma screens in reception etc.
These machines use the Intel 915G integrated video card which frustrated people by being advertised as Vista-ready (supporting Aero Glass transparency in the Vista Beta) but then having Aero support withdrawn completely. Since there is no WDDM driver for this chip, we need to use the XDDM (XP Display Driver Model) driver. For some laptops Windows Update will detect and install this driver, but not in this case. Intel also seem to hide it away on their download site. It can be found by navigating:
Graphics > Laptop Graphics Controllers > Mobile Intel® 915GM/GMS, 910GML Express Chipset Family > Windows XP Professional
…or directly from here.
The SoundMAX Integrated Audio is detected and installed by Windows Update, but if you’re maintaining a WIM image or building an unattended install you’ll need to isolate the driver files. These are also available at Microsoft Update Catalog – search for SoundMAX Integrated Digital Audio, order by date and look for the Windows 7 driver published on 10/31/2006 version 5.12.01.7010.