|Generic question, generic answer. More details, better answer.|
If your drive is full, you need to make more space. That leaves you 2 options, delete items and add additional drive space.
For the short term, Clean out cookies, internet history, temporary internet files, etc. using 'Disk Clean Up'. Clean out old emails, spam folders, AND deleted emails folders. Go over My Documents folder for really old files you no longer need. Empty recycle been. Uninstall any program (Add/Remove Programs) you do not or no longer use. Reduce System restore space used from around 15% to 10% or 12% of disk space which will only remove some of the older restore points. After you have opened up some room, do a defrag to open up a little more (It will complain if you have less than 15%, but it will work if you have around 10% typically.
For the long term, you need more disk space. If this is a laptop, or if you are not willing to open up your case, purchase a USB external hard drive and move many files (important but not often used) to this drive. If you have a desktop and are willing to open up your case for a very simple operation, purchase an internal hard drive and install it, then move over ALL of your personal to the newer and larger drive, using the original drive for Windows OS and your programs. If you cut and paste your My Documents folder to the new drive, make a short cut to it and put that where the old one was on the 'C' drive AND on your desktop so it will be easy to find (move and make other folders and subfolders as needed to organize. If you have an older computer, assume you will need a PATA or IDE drive rather than a SATA drive when purchasing and set the jumpers to 'Slave' since you probably have 2 IDE cables, one with your hard drive as master, and one with your optical drive as master (this is perfect without changing) and attach it to one of the Middle IDE connectors plus an auxiliary power connector and format.
If you need more specifics, supply more specifics. This is definitely much more than most will give you considering what little you supplied us.
You have to be a little bit crazy to keep you from going insane.
If all else fails, read instructions.