Windows 7 generic USB DAC driver and bit accuaracy

I am new to this forum and hope I can be helped with an issue that has me completely confused.
Over the last few years I have custom built several media computers, all based on Windows platfroms. The OS installed have included XP, XP Media Center, Vista and now Windows 7. I use Foobar, Media Monkey and Windows Media Player interchangeably
On two of these computers I initially installed XP, and recently upgraded one to Vista, the other to Windows 7.
Both of these were connected to reasonably high quality pre-pros (A Lexicon MC-8 in the one case, and a Levinson 40 in the other)
Connections were via USB to SPDIF converters. These converters used either the generic Windows USB Dac or USB headphone driver.
In both cases, the XP systems provided bit accurate playback, confirmed by both DTS music tracks and HDCD music tracks.
Since installing the Vista and Windows 7 upgrades, these systems are no longer bit accurate unless I use the ASIO4all drivers. When using the Asio4all, I cannot get true gapless playback on files that were recorded gapless, so I would prefer to use the generic Windows drivers. I was under the impression that Windows 7 and Vista were supposed to have improved audio performance.
My questions are if there is any way the Vista and Windows 7 drivers can be changed to allow bit accurate playback, and why the Vista and 7 systems are not bit accurate while the XP systems were.