Windows eats up space! How do I get back space?

Hi
I have low space on my C: drive which is a problem now since it seems as though Lightroom needs to use some space there to work. Anyway, is Windows 7 really supposed to take up so much space? The windows map is 15 Gb big!!! What takes so much space and how can I make windows take up less space? Seems as though there should be ways to optimize this because if feels like windows just grows and grows!
All of your help is very very appreciated!
Thanks in advance!

what happens after hibernation is turned off? Why does it relieve space and when it's on, why does it take up plenty of space?
Best regards

Yes, 15 Gb is fine, and disable hibernation if you don't need\use it (use sleep instead).

And check the size of the pagefile (some reading), use a min\max setting, with a low min, and all will work well. & move any large user files (videos etc) to another drive.

Wont' harm experimenting with hibernation & pagefile (as opposite to attempting any windows shrinking, which will most likely result in many, many posts here asking for help!)

Sorry, I'm a noob but I don't seem to be able to get admin permission to disable hibernate feature with the command prompt. I followed this instruction but it seem as though I could not really do it as such. I have only one user in the computer so I don't even know why I automatically cannot have full control...

haha, it was not harder than that! Thank you guys, you are the best!