ok by memory i assume you mean Hard drive space.. did you keep you pc the way it was when you bought it? or did you do like you should have done and format the sucker first thing.. pcs ( from best buy and circut city, places like that) come loaded with a bunch of bullchocolate software and usually have a backup partition... what i recommend (which takes some time) is to make a new partition with the free space you have, move all files you want to keep to that or to dvds and format the main partition with windows on it...  then reinstall your OS of choice and move your stuff back and remove the partition....  i keep my pc files sorted very well so i know where everything is and i use mutiple HDs so re doing the OS is a snap.. use a tool like nlite for XP or vlite for Vista and its make it even easier......  that's the hard way and be careful with partitions or you could lose all your data, but i guarantee you will get your space back if you don't mess up

still 40 gig though?  sure you don't have a folder hidden somewhere?