All your files, including Windows and programs and personal files, will sit on that 160Gb hard drive.
The more you download from the internet and store on that drive, the quicker it will fill up. Videos are the largest files so if you have been downloading video files then that will take up a lot of space.
Note that while you may have a 160Gb hard drive it may be divied up into sections called "partitions". One of those will be C, the others could be D or E for example (you will also have a CD/DVD drive that will be one of these letters as well).
Your C may be full, but you may have a D or E that is empty or nearly empty.
You need to find out:
If your 160Gb hard drive is divided up into partitions (C, D, E etc).
How big each partition is, and how much space is free on which
How many large video files you have on C, and if you can delete some. Your video files will probably be in My Videos but may also be in My Documents or My Pictures.
DONT just delete programs. They will gain you very little space, and may well stop things working.