Generally it isn't that good an idea to compress your root partition (C) because that's where your operating system normally is located. You can compress your other partitions. D, E, etc.. I use to do it way back in the days of Stacker (anyone remember that?) but with the prices on drives so low today there really isn't much need. The technology is safe enough...however if something does go wrong (rare, but it can) you run the risk of losing EVERYTHING on that partition. I speak from experience, happened to me once) I haven't used it for years, but way back when the method used was in effect to write one giant file. So that compressed volume was really a single compressed file with markers indicating where your actual files stated and ended. If something bad happens you could lose everything in the volume from the point of the corruption to the end of the volume. So your risk is much greater if something bad happens. Instead of losing a file, you can lose some or all of the files in the compressed partition, assuming they still do it that way.

So OK if want to risk it and have good backups. If not, I would go with a additional drive. I don't think you can compress NTFS drives, if my memory serves, it only works with FAT 32 or earlier.