Here's a brief description of what happened to me two days earlier (this is not helping, but proving that something is not right):
- started my notebook the morning and started aMule to download some things (ed2k + kad connected)
- went to work
- came back from work
- black screen on the notebook, because of power-saving function of xubuntu turned screen off
- so... moved the mouse
- notebook starts reading from hard disk like crazy and doesn't really respond
- one minute later desktop is visible and applet showing both RAM and SWAP are full
- one minute later xubuntu kills aMule because "Out of Memory" was detected
- memory is freed and notebook is responding normally again

EDIT: Just remembered some important detail: while I was at work two 350 MB files got completed.
The notebook has 512 MB RAM.