I have only 20gb space for C drive and 256 for other drive but my laptop is under warranty i have visited to service centre of Acer but they inform i have to pay to increase space for C drive and my keyboard is also not working properly

If C drive represents your whole hard drive, the way to increase the actual storage space is to get a bigger hard dribe, or get an external option that you can connect via USB to store your data. If you only want to increase free space, you need to delete/remove old data and programs. Music files, pictures and videos and old programs uses up a large amount of disk space. However, since you might need to add or save new ones again, getting a bigger or extra drive is still best which means, you'll need to purchase one and this is not covered by your warranty. The warranty will cover repairs and replacement of same specifications, but not upgrades. As for the keyboard, if you are using a desktop computer, test your keyboard on another USB port or on another computer, If it has same issue, you'll need to get a new one. If this is a laptop, try an external USB keyboard and if it works, remove the battery and charger and with the laptop turned off, press the power button for about 10 seconds and check if that helps. If not, the keyboard may need to be replaced which should be covered by warranty.