spotlight constantly reindexing

Running 10.5.8, spotlight shows with a dot and when I click on it I get a bar with the message "Indexing Imac", and "About -2147483648 hours remaining". I tried disabling indexing from the command line (sudo mdutil -E /) but got the response

> Error: unable to perform operation. (-1)

> Indexing enabled.

Then I tried dragging the hard disk onto privacy and then off, as given in online instructions - the hard disk went in and disappeared so I was not able to select it - and it also is removed from Finder!

There are 2 mdworker processes, one owned by _spotlight and one by another user. Is this normal? Neither one is using much CPU, the machine is maybe a little slower than it used to be but not seriously slower as it would be if it were really doing anything. The machine has been running for 4 days, and has been rebooted several times since it started doing this. The other user is not signed on.

Thanks