It doesn't. Well, it does, but it mangles some files in XP that cause it to boot slowly, and it breaks any Dev Studio 2005 you might have had installed in XP well and truly, VS throwing out random errors in the MS dlls when debugging stuff. They say, "never install beta code on production systems", I'd add RCx code to that list too. Oh, also if you had Vista Beta 2 installed as a multiboot system (that didn't mangle things), and install RC1 over the top of it, it looses the multiboot because it copies ALL existing Windows files into a new directory called "Windows-Old" (or something), including ntldr and all the other NT/XP boot files.. doh. Thats relatively easily fixed (copy back all the XP boot files to the root and just reboot) but a PITA.