You can direct download the installer for Photoshop CS 5.1 here Download Adobe CS5.5 Trials: Direct Links (no Assistant / Manager) | ProDesignTools
You must pay careful attention to the download instructions, or the links won't work.
I assume your serial number is for the standalone product, but if your Photoshop is part of a Suite, download the proper suite.
Go ahead and deactivate to be safe.
Gene