Windows audio
You can find tons of complains about XP sound quality on the internet.
The K-mixer is the culprit.
The mixing and sample rate conversion is done in integer.
Probably one of the design goals was to be frugal on system resources as the hardware at that time (1999) didn't had the power of today's PCs.
Despite all these complains, it is possible to get bit perfect output from XP.
From Vista on Windows has a redesigned audio stack.
The DSP (Digital Signal Processing like sample rate conversion, mixing, etc) is done in 32 bits float.
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
Win7 has the same audio architecture as Vista.
Although the new sample rate converter and mixer are probably not delivering the best possible sound quality, it certainly is an improvement compared with XP.
For best out of the box experience, Win 7 is to be preferred.
In case of a dedicated audio PC both XP and Win7 can be configured to deliver bit perfect output.