How to increase the size of drive c for windows 7

Is there more unallocated space on the drive? Then goto disk management, click on the c partition and extend it. If there is no more space on the drive, you need to buy a bigger one and clone the old one to the new, for example with acronis.

  You can compress the drive if it is NTFS file-system. But you will not be able to gain much, a few GB's at most.

In Explorer, right click the drive -> Properties , look for Compress drive tick box at bottom, and tick it. Click apply.

It will take a while, so do it when you not goin to need the PC for an hour to several hours.

But like noidea_77 said, it is best to buy a bigger GB drive.

not too much help you can get from build-in disk management, you can extend the C only when you have unallocated space behind it. So 3rd party software is the better choice, but backup your data before you partition is always recommended. Here I recommend AOMEI Partition Assistant which works fine for me, you can find the software from PCworld: http://www.pcworld.com/article/257248/aomei_partition_a... they speak highly of it.

If you have other partititions they can be re-sized or deleted to create unallocated space for 'C' to be expanded into, but best done with free downloadable software. I use Easeus Partition Master as it's easy to use http://download.cnet.com/EaseUS-Partition-Master-Home-E...