Well, you can do it with Debugging Tools for Windows, for which support has been added in Visual Studio 2012. You should also have Windows DDK 8.0 installed, after VS2012 is. Then, you should install "X86 Debuggers And Tools-x86_en-us.msi" package on the target (XP) machine, configure firewalls properly (better disable completely) and attach using new "Windows User-Mode Transport" (WinDDK adds it). Now you can do usual F10/F11/Shift-F11 stepping.
However, there's one rotten apple in a bunch - Windows Debugger works W-A-A-Y slow, stepping from line to line with 5 to 10 seconds delays... For me, it's unacceptable, so I'm still debugging in VS2010 with all its bugs (ha-ha).