XP versus Vista

There are claims that Vista sounds better then XP.

Windows Vista features a completely re-written audio stack designed to provide low-latency 32-bit floating point audio, higher-quality digital signal processing, bit-for-bit sample level accuracy, up to 144dB of dynamic range.

Source: Wikipedia

As usal in audio, this claim raised controversy: http://www.hydrogenaudio.org

XP can deliver bit perfect output if:

volume control is set to max,
1 and only one audio stream is playing,
the sample rate is supported by the audio device,
the bit depth is supported by the audio device.

If any of these conditions is not met, the K-mixer comes in to play.

A common way to overcome the design flaws of XP is to use ASIO (Audio Stream Input/Output) drivers as they communicate directly with the sound card by passing the audio part of XP.

Of course, your sound card must support ASIO to do so.

Aqvox recommends to use ASIO drivers with XP in combination with their USB DAC. They also supply an installation instruction.
Benchmark recommends NOT to use ASIO drivers in combination with their USB DAC.

Beware of 'Custom' Drivers!

Until now, high-resolution USB audio devices required 'custom' drivers. These drivers may compromise the stability of the operating system, and may cause conflicts with other installed devices. In addition, custom drivers usually consume more system resources (memory and CPU) than native solutions.
It is also interesting that many of the ASIO high-resolution USB devices we tested failed to deliver bit-transparent audio. In contrast, many of the native USB audio devices delivered bit-transparent audio. Our tests show that custom drivers do not guarantee bit-transparent data transfers, and that bit- transparency can be achieved without custom drivers.

Source: Benchmark Media

Both agree you don’t need this stuff in Vista.

Compared with XP, Vista has

better USB audio driver,
better sample rate conversion (32 bits float instead of 16 bit integer and better algorithms)
Exclusive mode (WASAPI) allowing an application to send the audio straight to the audio device bypassing the mixer.

I think you better upgrade to Vista for best out of the box audio performance.