well, you just need to resize your drive : GParted is done for that 
On another hand, you should try something else :
Run the livecd
(re)create a partition on the 80 go drive (ext3 or fat32, i dont know what you prefer/need)
when this is done
Mount each partition like this:
mount /dev/hda1 /mnt/custom ### i suppose the 20go drive has one partition which is hda1
mount /dev/hdb1 /mnt/windows ### i suppose the 80go drive has one partition which is hdb1
Then open Midnight COmmander : open Xterm and type "mc"
on the left go to /mnt/custom
switch to the right (using "tab" key) and go to /mnt/windows
go back to the left and select all files you wanna copy using the "inser" key
Press F5
BE CAREFULL here : it seems not do anything BECAUSE the compter calculate how much files there is to copy
When it give you the hand back , click ok
F10 to quit and then umount all and reboot.
It should make the trick 
Larry
GParted-project Admin
Former GParted-LiveCD maintainer (2007)