... it is possible that you have so much data on the partition that there is no room to shrink it. You may have to delete some files. You may also need to run disk maintenance like scandisk and defrag.
Windows has built-in limitations - is has been deliberately hobbled in a number of ways which gnu/linux users learn to dread. So it is hard to tell if it is just that the disk-manager software has been hobbled and you need to buy a better licence to free it up or something. You could try using diskpart.exe instead:
It is more usual though, just to use the linux installer to resize the partition for you.
The first thing is not to install fedora 10 - it will soon be unsupported and will neet a very large amount of updates to be secure after you've installed it. This would pretty much remove the disadvantage of getting an up to date ISO.
If downloading a DVD ISO is daunting, find a LUG close to you or try a CD-based distro like Ubuntu instead.
Back up your data.
Use linux to resize the native ntfs partition, not Vista. That way you bypass any MS funniness.
To bypass all MS funniness, of course, just install to the whole disk