there are various options available to you.
be ruthless on C:\ and re-locate all your User folders (like Docs, Pics, Music etc) to another drive like D:\ or E:\.
this is done by right clicking those User folders and selecting the Location tab.
also free up space by relocating the pagefile to another drive, delete the hibernation file, reducing Restore Points to 1% or disabling them altogether and reducing the Recycle Bin size.
but those are stop-gap measures, ideally you should backup the contents of D:\ and E:\ to some other medium, then delete those two partitions, that will combine those two 20GB's with the unallocated 830GB partition then you can expand C:\ to take up all that space.