|There are a few things to try first.|
Empty recycle bin.
Do a "disk cleanup", see help and support pages.
Might consider removing service pack files.
See temp folders and decide what to do with them. Many are wasted space.
Consider compress drive if you are not using encryption.
Do find files and search for large files. Decide what to do.
Might look at apps to find duplicate files.
Consider uninstalling programs that you don't use.
As for data it would have to be data not related to how a program is run. You can remove your word processing files like letters and fliers and such.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10