"I am currently using ROOTS III but I have less than 50 MB free drive space on this machine"

This is a tough problem. You will probably have to increase your disk storage sooner or later, so it probably ought to be now. The alternative is pretty risky; it amounts to creating a GEDCOM file of your ROOTS III data onto a floppy disk using the program R2GED that came with the Communications Utilities add-on for ROOTS III, erasing your ROOTS III installation to gain some additional disk space, installing UFT, and then importing the GEDCOM file using the instructions starting on page 271 of the UFT manual. If anything goes wrong in this process you could be in dire straits, so be sure to make a backup of your data before trying it!