Lenovo L My Lenovo T61p Has 4gig Of RAM But Only 3 Can Be Used Shown By Task Manager?

My Lenovo T61p has 4gig of RAM but only 3 can be used shown by task manager? - lenovo l

I have downloaded Windows Vista Ultimate with Service Pack 1, but I can not open 1gig RAM, after the download of SP1, I could see 4 GB of RAM on my computer, but the important thing l can not be used as by the 3gig RAM Task Manager, how can the release of these additional 1gig of RAM?