Storing movies on single hard drive vs raid 0

Hello,

Not sure how your question relates to the title but if I understand it, you currently have a 74GB hard drive that doesn't have that many files on it. Or do you not know how many files are on it and you are worried about space?
A 74GB will hold that amount of information but as to how many movies or pictures that is entirely dependent on the file sizes eg ripped DVD that haven't been compressed are 4-8GB each vs 700MB if compressed. Same with pictures, uncompressed RAW or bitmaps will take up much more space that JPGs. There is no real answer as to how many files it will hold.
If you deleted files from the drive, did you empty the recyle bin (assuming you are are using windows)? If not then this is most likely why you still have no free space. While I've heard of viruses corrupting files I haven't heard of them being able to actually mess with the hard drive (correct me if I'm wrong).
The other thing to look for is all the tempory files. If there a lot of them then this will also impact the free space on the drive eg does windows put its swap/page file on this drive.

Afraid without more specfic details I can't be much more help. Feel free to post the system specs, what AV, firewall, OS used etc.

Hope this is of some use to you