What is the simplest way of optimizing space in hard disk?

It's called disk defragmenting, Vista, Start Menu, All Programs, Accessories, System Tools, Disk Defragmenter It should be scheduled, unless you changed it, for 3 a.m. You can change that if you click Modify Schedule. You can also choose which disks to defragment with the Select Volumes button. If you want to defragment now, click Defragment Now, like you coulded have guessed that. It can take from a few minutes to a couple hours. Sorry, but I've forgotten how to do it with xp. Hope this helps!