Your license allows you to install Photoshop on a second machine, provided it is not used at the same time. During the CS5 install, you will be requested to enter the CS2 serial. Then you might need to reinstall the filters, and copy the presets from one machine to the other.
Thanks for that info. But now I'm wondering if it will work on Windows 7 or 8. I've been using XP. A guy at Staples says I can't. As a test, though, I installed my ancient PS6 onto a laptop Windows 7 and it works fine.