I recently did an experiment by dual booting Windows XP (64bit) and Windows 7 (64bit) on my laptop. Now, I'm not using the VG99 as an interface, but since I'm pretty sure this is a PC issue and not a VG-99 issue, I think it's still relevant info. Anyways, I'm using Ableton Live 8 and opening the same project on both machines. This project uses about 10 tracks; VG99, guitar synth track with a Ableton Live synth, clean vocals, vocoded vocals, 4 NI absynth/fm8 tracks for my keyboards, and 3 FX returns, and a master looper track. On top of that there is the usual compression and other polishing going on with each tracks (I output an essentially mastered mix to my PA). The PC is a Lenovo Intel core2duo 2ghz w/ 4gb ram with a MOTU 828mk2 and 2 USB midi keyboards. It's a fairly hefty work load.
XP wins hands down. Windows 7 had pops, crackles and full spaz episodes several times per minute with all this going on! Both of these are clean & fairly recent Windows installs that are used only for music production! XP isn't perfect with this setup either, I do have the occasional crackle, but it's fairly low volume and only happens about once every 15-30 minutes, usually. This is what prompted me to perform this test, but needless to say I'm not jumping ship just yet... maybe some day I can afford a Mac!
For what it's worth, I don't actually recommend XP 64bit unless your POSITIVE you can find all of your drivers for it... it's a bitchin OS that just unfortunately didn't pick up enough steam to get all the gear manufacturers on board with drivers. But if your gear does happen to support it I don't think you'll find a faster more stable platform for music production!