Not sure about the XP vs Vista/7. There's one way to use a VST EQ that is applied globally that works with DS or WASAPI but don't know of any way to get WASAPI globally used, would have to use ASIO for that probably BUT there's an alternative:
The only way which I know works to use a VST EQ applied globally to any sounds played on your computer and is loss-less streaming so it shouldn't result in any sound quality loss (I can confirm it sounded very much like using WASAPI with foobar2000) is to use:
1. Virtual Audio Cable (not free however, costs something 25~$50 or whatever but you can try it out for free if you can stand the annoying announcer spamming "trial" all the time haha)
2. A VSTHost to apply whatever EQ you want to use with your system, VSTHost is what I tested with and works fine
One downside with this particular setup though is that it:
a) Increases CPU cycles, but with modern CPUs there shouldn't be much of a performance issue/impact
b) It adds latency, you can control the "buffers" tho, the lower value you can use the lower latency but it'll start distort if you go too low, I could go down to like 525 whereas it was rather acceptable to me, even for gaming