20" iMac HD showing full, can't open Finder??

[DmbShn41]

I have a friends iMac (iMac 5,1) 20", 2.16Ghz, 2 GB RAM. Problem is at start up, after logging in. nothing loads. No icons on the desktop, no Dock bar, no menu bar. Firefox is set to load at startup, and once that loads, the menu bar comes back, and through that, I can navigate a few areas. One being the System Profile. The only SATA connection showing is a 250GB HD (factory), and available is showing 'zero kb'. He wants to save all of his files to an external HD, and then reload Snow Leopard, unfortunately because of the above mentioned issue, not able to. Anyone have any suggestions? I have had issues in the past on my iMac with rogue log files growing to 186 GB, but I can't even open Finder to navigate folders. Any help would be appreciated.

 

[Detrius]

If you have another Mac with FireWire available, you could connect it via FW Target Disk mode and explore for large files that way.

Alternatively, if you have an available external drive, you could put an operating system on it and boot from that.

Otherwise, if you're excessively familiar with Unix, you could boot into single-user mode and search for large files using du.