If you go into processes you can see whats using your memory up. it's in the right hand column. if you click on the top of the column you can sort them high > low.

you'll see something like

firefox.exe <username> 00 32,765KB

the figure at the end is how much memory it is using, the ones with higher numbers are the important ones and are most likely to be the ones that are thrashing the tits off your disk