Slow, Unresponsive Leopard

On my 2.0 Ghz C2D iMac the OS is just amazingly slow and unresponsive. Startup time is slow and the time it takes from entering password to both the dock and menu bar appearing takes ages (at least 5x the time compared to Tiger on my 400mhz G3!!!) when I drag something to the trash there is a couple of second delay, opening up programs seems slower than it should.

Why is this happening? I've tried pretty much everything, Onyx, I've archived and installed, and nothing has helped.

 

I don't have an ext hd big enough to backup, but I suppose I could do an number of DVDs.

Still my friend has the exact same issue on his 2.0 GHz C2D Macbook. The only thing we have in common is that we both used a CPU drop in disk that came with the computers, but I doubt that has an connection?

It's very frustrating, when I boot up I get a blue screen for 10-15 seconds (actually varies), then when I log in it takes probably 30 seconds for both the menubar and dock to be fully loaded (I have no apps at startup).

Finder seems slow, I click a folder like Application Support and it seems that it takes 5-10 seconds for the folders to appear.