How to install VS 2005 and MSDN for VS 2005 on XP without SP2

1. Navigate to the setup subfolder on the VS 2005 DVD and run VS 2005 as follows to skip all prerequisite checks - setup.exe /NO_BSLN_CHECK

-or-

1. Launch regedit.exe
2. Go to HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Windows
3. Change the CSDVersion value to be 512 or higher (0x0000200 in hexadecimal)
4. Close regedit.exe and install Visual Studio 2005
5. Rerun regedit.exe and change the CSDVersion value back to what it was originally

Once VS 2005 is installed, you will run into a separate block while trying to install MSDN.  To workaround this, you can use the same registry-based workaround listed above, or do the following:

1. Install Orca if you haven't already (you can get a copy here without downloading the whole Windows Installer PSDK)
2. Copy the contents of the VS 2005 DVD to your computer
3. Right-click on the file msdn.msi and choose Edit with Orca
4. Go to Launch condition table and remove the row with the following data in the condition column: "(VersionNT>501) OR (VersionNT=500  AND ServicePackLevel>3) OR (VersionNT=501  AND ServicePackLevel>1)    The minimum operating system requirement is Windows 2000 SP4,  Windows XP SP2, or Windows Server 2003"
5. Save and close msdn.msi
6. Install MSDN from the local copy

Note: after changing the CSDVersion registry key or modifying msdn.msi using Orca, you will need to use the following command line to install MSDN:  msiexec /i msdn.msi SETUP_EXE=yes

ref:
http://blogs.msdn.com/astebner/archive/2005/08/25/456611.aspx