Photoshop v.5 may simply be too old to accept the Vista patch. I seem to recall (although I can't find it, now) that Adobe had made an announcement that even the newest versions of Photoshop aren't ready for Vista yet ... but they're working on it. You may want to consider
The GIMP. Although there will be a little learning curve, and your plugins may not install in it, it's really a pretty robust, free Photoshop replacement.