Read this thread -» http://irshell.org/showthread.php?tid=86...88#pid6888

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?

The thread I linked to contains a few apps that people use to find out how their space on the HDD is used,

^^ lol it took me a minute to write all that and you beat me hibby.. but yeah try what hibby says before you format.. that's a last resort thing

idk why it would do that.. vista is dumb sometimes... maybe vista isn't deleteing stuff properly and see the data as still there even though its been assigned to be wrote over...  all the more reason to just use XP or dual boot like i do... bought this pc with vista and formatted it and put xp back.. now i have both just to mess with vista now and then

Try manually deleting.  c:\Users\*YourUserName*\AppData\Local\Temp
You can delete everything in this folder, and select *Skip this* for the
files that are in use.

I'm sure this is where your missing space is,