have you downloaded anything at all recently, particularly just before the problem occurred? Will the computer allow you to download anything now, if so try C Cleaner. Can you start in safe mode, if so do the same problems occur. Also do you have a copy of Hijack This? If you are able to run a HJT scan paste it in to the relevant forum on this site just to be sure you have no malware problems, in fact check that forum first because they prefer you to do HJT and other scans and post the results all together.