I believe when you are working in a domain environment your admin should have assigned a specific share drive for you to save your own documents to instead of saving them to your C: drive. Are you running windows XP or Vista? Either way you can RC on my computer(XP) or computer(Vista), then selct open and the dialog box will display oyu name to the lettered drive and you can move your items to that network drive to free up your HD. Hope this helps.
Assuming Windows XP since you did not specify version. Open My Computer, right click on C: drive, select Properties. There will be a button that says Disk Cleanup. This will help you perform some maintenance on your C: drive. But as others mention, you need to be careful as this is likely your system drive and you can make your machine inoperable if a bad choice is made.
You can easily free up disk space by deleting unnecessary temporary files and software applications which you think you don’t use anymore. You can also use third party software programs like tuneup utilities for maintaining the entire computer system.