Here’s how I installed it, taken from zend forums (http://www.zend.com/forums/index.php?t=msg&th=7855). This worked for me in Windows 7. Modify the paths below as needed for your installation of Windows 7.

1. Download and install the latest JRE for Windows: http://java.com/en/download/manual.jsp
2. Download and install WinRAR: http://rarlabs.com (be sure to install with Explorer shell extension).
3. Download Zend Studio 5.5.1
4. Right-click the Zend Studio install file and choose WinRAR > Extract Here.
5. Go into the folder Windowsresource (the newly-created Windows folder, NOT your system Windows folder).
6. Rename the “jre” folder to “jre backup”.
7. Copy your C:Program FilesJavajre6 folder into that resource folder and rename “jre6″ to “jre”.
8. Go back into that Windows folder and double-click ZendStudio-5_5_1.exe to install it.
9. When it is finished installing, copy your “jre backup” folder into your Zend Studio 5.5 installation folder.
10. Rename “jre” to “jre another backup”
11. Rename “jre backup” to “jre”
12. Run Zend Studio in compability mode for Windows Vista

Hope this helps.