Without doing it myself on my computer, it's hard for me to describe Plotter. If you are using XP, click on the start button and then on "Help and support". You should be presented with a list, to the right about half way down you should see "Undo changes with system restore". Click on that and it will give you a calendar, choose your restore point here. If it was last Sunday that things went wrong, I would choose the day before as the restore point. Then it's just a case of following the instructions it gives. It takes a short time 5-10 Min's to sort itself out, then the computer will need to be restarted for it to take effect. Once again, give it the chance to do it's thing and follow any on screen directions.
It won't harm your computer, so it's worth giving it a go.
Hope this helps.