You may have hard drive quota's turned on. There is an option to limit the amount of space users can use on a hard drive when Quota's are enabled. If you open up my computer and right click on the C: drive and choose properties, there should be a tab labeled quotas. You can enable and disable the option to use quota's there if you have admin rights. You can also change the size of the quota as well in those settings if you want to leave them enabled, but increase the size available to you.