Hi ,
I'm Dyari. Thanks for reaching out. I will be happy to assist you in this regard.
Windows Disk Management can only extend a partition if unallocated space is directly to the right of it. Based on the screenshot, C: is blocked by that 604 MB recovery partition, so you cannot extend C: using built-in tools unless something is moved. Otherwise, you need to use third-party partitioning tools like the free version of AOMEI Partition Assistant.
But, before touching partitions, create a full system image:
Click Start > Control Panel > Backup and Restore (Windows 7) > Create a system image > Choose
External hard drive (recommended) > Include C: & F: and let it complete. This creates a full image backup of everything.
You can also use a third-party cloning tool like the free version of Macrium Reflect to clone your hard disk completely. If something breaks, you can restore the entire disk.
Please let me know if you need further assistance.