Server will not boot into windows - I believe due to no system drive space?

I noticed that the system drive was completely out of space - so I rebooted the server thinking that during a reboot it might free up some space... but it would not boot into windows - it will go to recovery options - This is a 2008 R2 server - Poweredge R510 - Raid 5 - I was wondering if there was a way to possibly either clear space on the system drive pre-boot? or make the partition larger preboot? (without comprising any data) .. or any other options recommended to resolve this so that I can get back into windows to manage our system drive and free up space. Thanks