Steps to Build 64 bit libxml2 in VS 2008
1. Download latest libxml package from the web, and unzip it. Let the location be,
C:\libxml2-2.7.7-64\
2. After you unzip and extract in the above location, open Visual Studio 2008, Tools, x64 command prompt, not the default cmd prompt, and go the following directory.
C:\libxml2-2.7.7-64\win32
3.  The readme is quite helpful in getting the basics, but i will list the steps i followed to get it working.
4. use the cscript command to generate configure configure.msvc file. cscript uses the configure.js script to auto generate this file. 
You can use the following command for help. this basically throws lot of options at you, and whatever we mention here, gets loaded into the configure.msvc file we want to create. 
C:\libxml2-2.7.7-64\win32>cscript configure.js help 
5. The command i actually used, here we are mentioning that compiler is msvc, we dont want iconv, its a debug option, etc. Prefix here is very important, basically you can provide any directory such as ''pwd"here where it will generate all necessary files. 
C:\libxml2-2.7.7-64\win32>cscript configure.js iconv=no compiler=msvc cruntime=/
MDd debug=yes prefix=.
6. if you open the configure.msvc file after the above step, you will understand what i mentioned above.  Snippet below. the variable INCLUDE and LIB is to include other header files and library that you are dependent upon, suppose you want to compile with iconv, provide the directory where iconv is present, and also the library path. since i excluded it with iconv=no option, we dont see anything here. 
PREFIX=.
BINPREFIX=$(PREFIX)\bin
INCPREFIX=$(PREFIX)\include
LIBPREFIX=$(PREFIX)\lib
SOPREFIX=$(PREFIX)\lib
INCLUDE=$(INCLUDE);.
LIB=$(LIB);.
CRUNTIME=/MDd
VCMANIFEST=0

7. Run the following command as mentioned in Readme.txt

C:\libxml2-2.7.7-64\win32>nmake Makefile.msvc libxml

i got few compilation errors, starting at

..\dict.c(280) : error C2061: syntax error : identifier 'xmlDictComputeBigKey'

open this file, and add the following, snippet
#elif defined(WIN32)
typedef unsigned __int32 uint32_t;
#elif defined(_WIN64) // add these 2 lines
typedef unsigned __int32 uint32_t; // added
8. Re run the above command and your libraries are built. check the winnt folder, it would have created bin.msvc, int.msvc , etc. Your DLL and LIB files will be in bin.msvc. the makefile in the same directory is very neatly written, so you can refer it for other make options.

Hope it was helpful.