System Hesistation

I am experiencing a complete system hesistation and have run virus scan and a rescue disk and found nothing. It starts occurring after the computer has been on for a while and if I restart it works fine for a while then hesistates. If an email has music it stutters or if a game talks it stutters, if I try and send an email it takes longer than normal sometimes. Also, when it starts hesistating my mouse does not work properly either. I can click on it and nothing happens or move it and it takes a few seconds for the pointer to move and it moves weird. It is all very frustrating and Dell's answer was I have the ebug virus, which I don't and they said I would probably be calling them back and be ready to reformat my hard drive which I really don't want to do. If anyone has any suggestions. I would GREATLY appreciate it. Thanks