Try LW 4.8.1 as suggested in PM. I have a suspicion the problem is related to the method in how you installed Leopard OSX 10.5.
BTW what happened when you set up a new account? Same problem?
You might have to end up reinstalling Leopard, which is not as painful as it sounds really. Only painful part is doing the updates.
Also, do you have any partitions or 2nd drives on your computer?