They are several ways to do this, but before I should tell them to you, I would like to inform you that not only copying files into you're computer may make you're HDD full, space can be filled up even because of spyware, viruses and other malware threats. Now here are some options for you're question:
1. Delete files, uninstall software/games you don't use anymore, a movie by example, can fill the space up from 900 MB to 3.0 or in some cases 4.0 GB, games also use a lot of space, some of them require same as a movie or they may be double-sized(8.0 GB) in some cases, so, be sure to uninstall the games that you don't play.
2.Use Windows Defragmenter option located in Start>All Programs>Accesories>System Tools>Disk Defragmenter by defragmenting you're hard disk can also help to free up some more space
3.Delete the System Restore cache or even the entirely option, but be careful, use this with CAUTION, if something in the future will make you're system have problems, you may no longer return back to a previous time before. Sending you're comoputer in a previous time if a problem persist, it usually is much easier and faster than installing Windows again to solve the problem, you may try this option as it does not affect you're computers current settings and files, even through I don't reccomended.
4.Buy a larger HDD. Larger Hard Disks are harder to be filled, a HDD of let's say 80 GB will be easy to fill, but one of 500 GB, it would be a lot harder, if you buy a new one could still use you're current HDD, even more, by adding a new one the more space will add. Example, you have a HDD of 80 GB if you add one of 400 GB near the one of 80 GB the result will be that you will have 480 GB. Hope the information helps.