Just a "thought" - is your voice recognition turned "on" ?? - turn if "off".
I have an HP notebook PC with Windows XP, Norton SystemWorks 2003 and MS Office 2003. When using MS Word, my computer randomly inserts the letter "c" in a document while I'm typing it. When going back to where it inserted a letter "c", it will insert yet more of the letter "c" in the word I'm correcting. I keep the virus definitions up to date in Norton Anti-Virus and run a full virus scan every couple of weeks. No viruses have been detected. I also run Spybot - Search and Destroy regularly. I have not found any mention on the Internet of this problem. Does anyone have any suggestions of what is causing this problem?