Maybe the system protection is not turned off. Check it. System properties >; System protection > protection settings to see if the protection setting status of drive C is "off". If not, change it to Close, and then click OK. After that, you can compress disk C in disk management and create a new partition.

Drive C refers to one of the main partitions of computer hard disk, which is generally used for storing or installing systems. For a single operating system installed on the local hard disk, it is the default local system boot hard disk. Most of the files on disk C are mainly composed of system folders such as DocumentsandSettings, Windows and ProgramFiles. ProgramFiles folder is generally the default location for installing software, but it is also the location of viruses, so drive C should be strictly protected. Drive C is extremely important for the single operating system of local hard disk, so try not to put drive C when storing data at ordinary times. By default, the personal directory that needs to be removed from drive C is: C: \ DocumentsSandSettings \ Your Login Account \ Documents.