Cannot copy or unzip ".zip" files

I am running 10.5.8 Dual 2 PPC G5 and recently cannot copy or unzip ".zip" files.

When I try to unzip I get the "unable to unarchive (error 1 - operation not permitted)" message. I've tried archive utility and stuffit expander and the terminal. None work.

When I try to duplicate I get "The finder cannot complete the operation because some data in "filename.zip" could not be read or written. (Error code -35). The copy progress window remains and I relaunch the finder to get rid of it. Same thing when I try to copy and paste them.

I've tried to check the console, as per some threads, to see if there might be something jamming it up. I only get the rainbow wheel and it sometimes locks up the machine for a few minutes when I try to scroll through or click on anything else in there. Makes me think there's a larger issue, perhaps?

I've tried some basic stuff like permission repairs and restarts with no change.

The source of the zip files vary. But other machines on the LAN can open the same ones with no trouble. They vary from software downloads to customer files.

All other operations on the machine are functioning normally and efficiently, from simple tasks to processor heavy ones.

Any help would be greatly appreciated. If anyone needs any more info to assess I'll be happy to provide it if I can.

Thank you very much.