I recently began running out of space on my C: Drive (120GB SSD). It turned out that 13GB of this was used by HyberFile.sys (which is used for hibernation).

To reduce this, I opened an administrator command prompt and ran

PowerCfg.exe /HIBERNATE /SIZE 50 (where this size is a percentage of the machines memory).

See http://www.ehow.com/how_8617946_reduce-size-hiberfilsys.html for further details.