Here I will show you how to install NS2 on Windows 7 (32-bit | 64-bit):
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Important:Your User Name Shouldn't Contain any SPACE(e.g: ram sahu).If contain then change it.
Step 1:

•  Go to these link and download the three .rar files and place them in a single folder.
• Link for part 1(100Mb):NS 2.35-Part1
• Link for part 2(100Mb):NS 2.35-Part2
• Link for part 3(19.49Mb):NS 2.35-Part3 

Step 2: 

• Now Extract the NS 2.35.part1 only.It will extract the part2 an part3 automatically.
• Note:
• Extract after complete download
• Don't Extract the part2 and part3 

Step 3:

•  After extraction you'll get a folder named NS 2.35
• Inside this folder you get cygwin setup , .bashrc etc.. 
• Now install the setup.  Choose install from local directory

 

• Press Next
• Don't change the default Root Directory
• In Local Package Rpository field browse to: NS 2.35/nslocal/release 

• Click Next

• Click on the Circle

• After clicking the circle the default turn to install

  

• After this press Next it will install the necessary packages for NS2
• check create a desktop shortcut and start menu shortcut
• Then installation finished.:)p
• Now you are halfway done !!!
• Now You'll get a screen like this:

• Now create a folder Named Noureddine in C:\cygwin\home
• Now copy the ns-allinone-2.35-RC7avecxgraph.rar and Extract there. You'll get  ns-allinone-2.35-RC7 folder.
• Now cpoy the .bashrc file from the downloaded Extract folder
• Now go to the "C:\cygwin\home\prittam" here "prittam" indicate my user name in your case it will be your user name.And replace(i.e paste) the .bashrc file that you copied.
• Now open the Cygwin command window which is on your desktop and type source .bashrc
• Now type ns  "$" change to "%"

• Now type ns-version it will show like:

• Now press exit.

• Now type startxwin you'll get a window like this:

• Now type nam you'll get a screen:

• Now go to file and quit the nam window
• Now hold the main tab and drag your mouse to quit from Cygwin window(the white window)
• And exit from 'cygwin command' window (type exit)
• Important:
• go to Taskmanager(Alt+Ctrl+Del) and goto process tab and search for XWin.exe

• End process
• If you not do this when you press startxwin in Cygwin command window for the second time then you get this:

• Now open the cygwin command  window startxwin and type ns yourfilename.tcl then nam file is generated and type nam yournamfile.nam
• For my case:

• Write your code in notepad++ and save with .tcl extension

----------------------------!!!!!!!!!!!!!!!!!!!!!!!!Congragulation!!!!!!!!!!!!!!!!!!!-------------------------------

Any query please do write as comment or send mail "prittam.nanda@gmail.com"
Thank u