HP ProLiant ML350 G6 driver problems

Windows uses a number of factors to decide what is the "best" driver, such as hardware ID matching (INFs that match more specific hardware IDs are "better"), driver date, driver version, and whether the driver is signed. How Setup Selects Drivers has the details if you're interested.

As for how to remove the newer driver, if it has an installer listed in Add/Remove Programs, then by all means use that. If not, you should be able to manually choose the older driver by right-clicking on the device in Device Manager, selecting "Update Driver...", "Install from a list or specific location", "Don't search. I will choose the driver to install", etc. Or did you already try that? (As well as selecting "Uninstall..."?)

If all else fails, you might have some success using the devcon command that ships with the Windows Driver Kit. You can remove a driver package using devcon dp_delete, or force Windows to choose a different driver using devcon update. There's also an older version of devcon available for direct download (the WDK is pretty large).

If you resort to manually deleting the driver's .inf file from %SystemRoot%\inf, be sure to delete the corresponding .pnf file as well, because Windows uses that to cache some/all of the .inf information.

Try using DevCon, a Microsoft utility.

The DevCon utility is a command-line utility that acts as an alternative to Device Manager. Using DevCon, you can enable, disable, restart, update, remove, and query individual devices or groups of devices.

As long as you unpack your drivers into a default search path, you could call a rescan to capture all the devices not installed initially.