Recently Google has changed the position of adb. It was in the tools/adb and now there is a new directory called platform-tools.
If you have any problem with android SDK and you haven't change any configuration, maybe the problem come from Google.
Check this to see if you have this problem.
1. Click on Windows -> preferences (or Eclipse preferences)
2. Click on "Android".
If you see a warning message warning you that it cannot find adb, then maybe you can fix with the follow method.
Changing the position of ADB.
1. Go to Help -> check for updates. It will open a new message box.
2. Select all the android package and install them.
After this, you will be able to open the "Android SDK and AVD manager".
1. Go to Windows -> Android SDK and AVN manager
2. Go to "Available packages" and select "Android Repository"
3. There should be something called "Android SDK platform-tools" (the actual version is "revision 3"). Select it and press "Install selected".
4. Restart Eclipse
This is also the way to update new packaged (for example, for the new SDK for android 3.0). Check it sometimes to be updated.
Like always, if you have any problem, just put any comment.