Very high memory use for no obvious reason

For the last couple of days my SonyVaio has been consuming massive amounts of memory - I have 4GB of physical memory and it's using 90% and more much of the time, stopping the computer running properly - especially online. The big user on the processus list was svchost at 140,000K or so. I read a forum and disabled superfetch and that pulled its usage right down but memory use is still over 90% - any ideas ? solutions ?! thanks