What version of Windows are you using?
If you are using Windows 7 or 8, you can try to process this low disk space problem in three ways:
1). Delete some unnecessary files or programs on your C drive to free the drive space.
You can have many ways to free up your C drive space, like deleting some useless files/folders, uninstall some unnecessary programs and running the Disk Cleanup tool.
2). Extend your C drive in Disk Management
Back up everything important on another drive, shrink your rest partition with enough free space and extend your C drive by right clicking it in Disk Management.
3). Apply some partition resizing tools.
If you are not so familiar about the partition resizing tuffs, you can directly use some partition resizing tools like: GParted, IM-Magic Partition Resizer Free, Active Partition Master and more.
PS: Always back up everything important well before resizing partition in case of data loss.