Drivers for the Dell OptiPlex 990 can be downloaded below, Windows 10 is probably as higher version as you would wan to install on this desktop. If you are going to attempt Windows 11then you will have to use this work around for the lack of a TPM 2.0 module: Installing Windows 11 on a computer that doesn't meet the requirements.
Graphics Drivers:
Sound Drivers:
Network Drivers:
Chipset Drivers:
