Sergey's suggestion of a 3rd party tool is good. They compress to about 20% of original size (depending on your data) and are faster than native SQL backup. Then you can fit 4 backups on your drive.

Or buy another or bigger drive. Shouldn't cost much for the size you're working with.

Also, as George brought up, copying off to another location not only frees up the space, but provides protection by having your backup in another location, especially if your database is on the same drive (bad).