taimatsu no monogatari

My computer. It has free space on its hard disk. There is nothing obviously wrong with it. And yet it is grindingly, drearily slow, spending enormous amounts of time 'waiting for I/O' when I do more than the very bare minimum with it. This situation is new in the past month or so. I am running Debian Linux (etch) with KDE. How do I sort this out? I am sick of waiting minutes to open a KMenu item.