It is likely that you have the following
1) Huge Log Files
2) Incorrect Permissions
3) Damaged Files
To correct them
1) Open Applications/Utilities/Console - clear logs
2) Open Applications/Utilities/Disk Utility - repair permissions
3) Open Applications/Utilities/Disk Utility - Verify Disk
Also, considering your system, you should be running Tiger for optimal speed and efficiency.
TEG