The onboard Realtek sound chip should support at least 24/96 unless it is really ancient. If you go into Control Panel > Sound and check Properties on the Realtek Digital Output device, the Supported Formats tab will listed the sample rates that the device can output.
To my knowledge, using the Test button on each sample rate will bypass the internal Windows mixer. However, unless you are using a Windows Mixer-bypassing driver such as WASAPI (Windows Vista and 7) or ASIO and a player that supports it, all sample rates will be downsampled to either 44.1KHz or 48KHz when playing music.
WASAPI is the only driver that I am absolutely convinced completely bypasses the Windows Mixer, as evidenced by the attempt to play higher bit depths or sample rates than the DAC can support returning a hardware unsupported format error.
I have also never seen a 24/96 or 24/192 DAC that couldn't output 24/96 through the S/PDIF optical output.
Steve