Ok, previously I've got XP. I din't took note of how much space is used. I tried to do a clean install on Vista but for some unkown reason, the disk wont boot up. So, I just installed Vista from the AutoPlay feature on the DVD-ROM drive. Well, most of my stuffs were deleted but I heard that the core installation takes 8GB but when I check on how much disk space I've got, It said I got 19GB of used space. By time, Its increasing rapidly. I did use the Disk cleanup feature and delete the shadow files something like that, but it dint helped. So can someone help me out here? Where to check or what software could I use ? Thanks.

here are my suggestions:

advanced windows care
ccleaner
glary utilities

just google them, maybe they will help......