so it's due to an issue within the build tools we're using as part of VS2008 SP1 where it puts 9.0.21022.8 in the manifest as the version for msvcr90.dll irrespective of what is found during the build process (as per here
the issue isn't limited to XP and just relates to the recent 5.64 / 5.7 builds only if the 9.0.21022.8 version of msvcr90.dll is not already present in the WinSxS location (or appropriate place on other OSes).
so the solution is going to be changing the version used in the dll and our manifests (before going through the VS2008 tool) to reference 9.0.21022.8 even though we're actually using 9.0.30729.6161. then that will get it working as expected when the redistributeables are not present on the rest of the machine and still allows the OS to find the appropriate version to use as needed (as i think is what you're seeing Aminifu - side-by-side dll dependencies are loads of fun *joy* ).
or maybe not. looks like we'll have to go with the patching of winamp.exe option as just changing everything to reference 9.0.21022.8 instead of 9.0.30729.6161 didn't work.