You can compress the drive if it is NTFS file-system. But you will not be able to gain much, a few GB's at most.
In Explorer, right click the drive -> Properties , look for Compress drive tick box at bottom, and tick it. Click apply.
It will take a while, so do it when you not goin to need the PC for an hour to several hours.
But like noidea_77 said, it is best to buy a bigger GB drive.