hard drive inaccessible under Vista/Xp but not dos

Hi, I'm trying to fix a laptop for my daughter's friend. It won't boot into Vista all the way. Hangs up at a black screen and the mouse pointer. Tried various things and one thing I found was Microsoft saying to install another copy of Vista next to it because it couldn't be fixed. So, I took out the hard drive and installed it as an external on my desktop. It shows up but says it can't access it. Also doesn't have the size of the hard drive. However, when I open a cmd prompt with admin privileges, I have access to the drive. Have tried taking permission of it. And I thought taking permissions was for the files, not for the drive itself? Would like to copy everything off it before I install Vista again. Plus, would like to know why it is doing this! Also ran chkdsk and it fixed some problems. Any ideas?