We don't know the content of your drive, either. It's best to buy a new one. IF this happens to be the one your Windows is on, start by deleting everything in your Documents folder that you don't use and uninstalling programs you haven't used in a while (or ever).
You have to be smart, though... you can't just go uninstalling stuff randomly. If it says "Mouse drivers" or "Graphics card" another computer part name, or the brand of your computer (which means that software came with it) then you need to leave it alone. Or anything Microsoft.
I think he's using the maxtor software to back up his system(This was how it was marketed). Then his question makes sense since he would want to delete the oldest backup.
I think that deleting the oldest backups would be fine as long as your backups are all full backups(not differentials or incremental). The only issue is that you would only be able to go back as far as you have backups for.