25GB is the recommended disk space for a window 8 consumer preview install, your C: drive just isn't big enough unless you wipe your OS that's all ready on it and you wouldn't want to do that, only install windows 8 on a test machine or as dual boot install in my opinion.
It looks like you have enough space on your E: drive where you could create a 25GB blank partition and then you could install Windows 8 consumer preview there, if not then get another or bigger hard drive.
You could also make a ghost image of your C: drive, get a bigger C: hard drive, reghost your C: drive back onto the new bigger hard drive and then create a blank partition on it of 25GB or bigger and install windows 8 there, that's above my head though and could be kind of hard to do.
hope something helps
First thing you need to get a backup image of your HDD.
Do you have an external HDD to keep it on?
This kind of partition adjustment can possibly fail and you will need it to restore your original partitions.
The other option, if you have a failure, is to re-install Win7 and Win8 and loose all your personal data.
How much do you want to add to the Win8 partition?
If you want to take 10GB from partition F, it would be the easiest.
Use Free Macrium Reflect to make a backup image, best to save the backup to a separate HDD.
This tutorial will explain how to use it: Imaging with free Macrium
Since Windows will not allow you to expand a partition to the left, you will need a third party app for that.
Partition Wizard is a highly recommended partition adjustment app.
Use the bootable CD version.
Another tutorial to explain how to use it.
PartitionWizard : Use the Bootable CD
This example is for getting 10GB free space from Partition F and adding it to your Win8 Partition C.
Do this in two separate steps.
1. Shrink partition F from the right side 10GB, this will leave a 10GB unallocated space between partitions F and C.
2. Extend the C Partition to the left to take the Unallocated Space.
Where do you want to get the space from for the C Partition?
This is just a quick explanation for this procedure, have a look at the links above, and ask any questions you have.