Does Rasterlink keep all the old jobs? If so you might want to go through the old jobs and delete them. Regardless you are running out of space on your system drive which is not good at all. As it gets lower other programs may also start having issues as well. Do you have just the one hard drive in the system? You might want to look at either replacing the current drive with a larger one and (depending on the system) add a second drive for just bulk storage (keep all client files, and other files on the new or if you already have a second drive D: drive).
One immediate measure you can take to help is to clean out all old junk. In the "My
Computer" or explorer view right click on your C: drive and choose Properties. On the screen that comes up click on the Disk Cleanup button. It will scan the drive and give you a list of things that it can remove to free up space. On that screen there is also a button "clean up system files". Go ahead and click that and it scans again then gives you additional things to cleanup like old
Windows Update files, queued error reporting files, service pack backup files, etc. You can safely remove most everything it offers to. Even temp files - it will only remove ones that have not been accessed in over a week.
Also can go to your control panel>add remove programs> and uninstall any programs you do not ever use (do not remove things like drivers though). There are also a couple of other things you can also do like in Power Settings, disable Hibernation. That will immediately free up 4GB if its enabled - your system will still be able to sleep unless you disable that as well - just will disable hibernation which creates a file the size of the amount of ram the system has. Hibernation is generally not needed on desktops (more useful on laptops).