help? why are my layers transparent when I nedd them to be opaque?

I have recently upgraded from PS v4 to v7 and something seems to have changed that I can't understand. I copy and paste an area, though the layer is normal and the opacity and fill settings are 100%, my pasted section is only semi opaque.

Also, when I use the clone tool, it is adding and darkening colour, rather than matching the flat colour. And when I use the paint bucket it is adding colour rather than replacing colour.

In v4 all these actions were replacing opaquely. Now I'm getting build ups transparently. Is there a setting a need to change?

Thanks for any help.
Cheers,
Gill

Try trashing your Photoshop Prefs., including com.adobe.Photoshop.plist, and run "Repair Permissions" from Disk Utility.