Why does my Toshiba Sattelite L300 laptop automatically write these letters? Is it a virus?

I bought this laptop from Argos nearly 10 months ago and it is still under 1-year basic guarantee which will end after 2 months. Few months ago, I began to notice that frequently while I'm writing (especially in Microsoft Word) and after finshing the sentence, sequences of c "ccc" and sometimes b "bbb" automatically written without pressing the letter keys. I'm sure that the keyboard is fine as this doesn't happen everytime I write. It could happen once every 2 days, and it could happen even more frequnetly (e.g. 5-10 times a day!). I was told that it must be a virus...What do you think? Has anybody gone through this painful, hateful experience? Any suggestions? I really hate this to happen, especially because I'm writing my PhD thesis and I don't want to be obsessed with reviewing every single paragraph in case that this might have happened...Serious and/or expert answers please! Thanks so much in advance.