To my understanding, Windows Defender is part of the Windows7 Operating System, and cannot be removed. It can be disabled, so you can run other anti-virus, or anti-malware suites such as Norton, McAfee, or Avast. This is echoed in the following article from PC World, which shows you how to DISABLE Defender here: Remove Windows Defender (As Long As Windows is Defended)
This occurred sometime around the release of Windows Vista in 2007, as in XP Defender or Microsoft Security Essentials as it was called for a period (2003-2005), was a downloadable stand-alone program that you could get for free from Microsoft. Once it was incorporated into the OS (Operating System) by Microsoft, it was no longer removable, but could be disabled as mentioned. Friends of mine at Microsoft told me the reason they did this was that so many home computer users would remove Defender/MSE in order to gain back performance or storage space on their hard drive--neither is a good reason to get rid of your primary software protecting you from a plethora of Internet nasties out there in cyberspace. This in turn caused hundreds of thousands of additional tech support calls to Microsoft and other computer manufacturers (Dell, HP, Acer/Gateway) and it was jointly decided that ALL computers, laptops, and tablets running Windows would from that point forward have built-in AV or MALWARE protection. This would ostensibly reduce the huge number of tech support phone calls received by MS and the computer makers, and indeed it did--as they saved millions of dollars by doing this. This is really a simple answer I gave you in first paragraph above, but I thought you'd like to know WHY this is the way it is. MS and the big computer makers felt that the Internet virus/malware threat was such a large problem, that computers must come with built-in protection. Turns out that if you take an older PC such as one with Win2k or XP which came with no virus/malware protection or if it did, and you removed it--then connect that PC to the Internet (virtually unprotected), tests back in 2006 showed that once a PC was plugged into the Internet, it only lasted 8 seconds before they were infected by viruses/malware! Newer tests in 2010 and later have revised this figure to about 5 seconds or so. This has been verified and documented heavily on the web and can be found in multiple referenced sources as as Wikipedia and Webopedia to name a few.