Go to http://www.google.com/ and type in Flashbuilder, then find the Adobe Flashbuilder Site. (right now Flashbuilder, formerly known as Flexbuilder, is in beta, so its URL will likely change in the near future...which is why you should find it in Google).
Once you are on the Adobe Flashbuilder site, click on the link that says Get the Flash Builder 4 beta 2 release , scroll down to where it says Flash Builder 4 Plugin for Eclipse, and click on the Download Flash Builder 4 Plugin for Windows link. Then click the Save button and save it to your hard drive. The file is called flashbuilder4_b2_plugin_win_100509.exe.
Double click on the flashbuilder4_b2_plugin_win_100509.exe file on your computer to install Flashbuilder. It will be installed to C:\Program Files (x86)\Adobe\Adobe Flash Builder Plug-in Beta 2 by default. During installation, click the box that says Plugin to another copy of Eclipse, then click the Choose button and browse to the directory where you've installed eclipse (C:\Program Files\eclipse).
VERIFY IT: Go to where your eclipse is installed (C:\Program Files\eclipse), double click on the dropins folder, and you will see that Flashbuilder has put a file there, com.adobe.flexbuilder.feature.core.nl1.link.