Below are the compiler types:
x86 on x86 (X86 native compiler): Allows you to create output files for x86 machines. This version of cl.exe runs as a 32-bit process, native on an x86 machine and under WOW64 on a 64-bit Widows operating system.
x64 on x64 (X64 native compiler): Allows you to create output files for x64. This version of cl.exe runs as a native process on an x64 machine.
1) Goto add remove programs and select visual studio.
2) click on remove visual studio 2005.
3) You will be taken to the visual studio 2005 setup.
Below is the screen shot:
4) Click next. you will be taken to the options for selecting any components.
5) select the x64 compiler option to install. You are all set to go.