Yes, probably. There are partitioning utilities that will work with a drive with OS and data. But to be safe, before using them, you should back up the complete drive to an external drive.
What you should do is defrag the drive, then image the drive to an external drive. I use a program called Acronis True Image - about $40. If you have a Western Digital drive, there's a free version tailored to WD drives on the WD website.
http://www.acronis.com/homecomputing/products/trueimage/
And there are other free backup programs.
This is insurance in case something goes wrong.
Then you can partition the disk. Again, I like Acronis:
http://www.acronis.com/homecomputing/products/diskdirector/
And, again there is free software to do the same thing.
Then you restore the data.