Read through http://www.google.com/search?&q=%22Unable+to+write+to+Disc+in+Drive+C%22&lr= to see discussions on other instances of this error.
Without any research, I'm inclined to say it's time for either a new hard disk or a new motherboard. It must be a rather new computer, so possibly you still have warranty on the hardware? Then it's the makers problem, not yours.
I have a desktop PC with a 2.8Ghz pentium4 processor 80gb harddrive 90% free 512 shared ram running windows 98se. I am having a continuous problem when my screen is suddenly blue with the message ? Unable to write to Disc in Drive C data or files may be lost. Press any key to continue? When a key is pressed the screen goes back to normal but can almost immediately return to the message and can do this repeatedly to such an extent that in the end you cannot return to a normal screen. Then when you reboot you get a message ? One or more of your drives may have developed bad sectors, press any key to run Scan Disk. The yellow line goes up slowly then at completion another message ? examining Drive C? then the disc surface scan comes on and begins to check the surface. If let go this scan takes 9 hours and as a matter of interest I have on about 6 occasions let it run overnite and no problems are found and the next startup is Okay. But it is only a matter of time before the blue screen pops up again and the whole process is repeated and this is now daily. One can pass over the scan and continue to load windows but looking at ones Email. Looking at the website of a reputable electronics outlet,o r evn playing solitaire while offline can bring this on. I have used spysweeper, Norton Virus Scan, Ad aware SE pro, Spybot S& D, and have removed any spyware, I regularly use Executive Software diskeeper, System Mechanic registry cleaner etc. I also use Zone Alarm firewall and Norton 2004 antivirus. I have no idea what is causing this problem except to say it is becoming worse every night. Any assistance would be appreciated. email@example.com