My PC runs very slow?

Hi, i have a PC with an AMD athlon processor 320 mb ram 995 hz with a 74 GB Hdd with 11.9 GB ocupied. It runs mostly painfully slow taking ages to complete just one task, eg 5 mins to open internet explorer window. Sometimes it seems to settle down and appear more or less normal but behaviour seems random but usually slugish for the first tasks after start up, even if left for a long time to settle down. Boot up seems to be about normal time. I have tried all kinds of antivirus programs and used to use cleandisk security but now use Ccleaner and regulary clear out and defrag. It appears to have got slower and slower over time (one month). Any suggestions how to cure it?