In order for someone to post a quick response, please answer: What type of computer is it (dell, sony)? What is the Operating System (98, XP, if XP - is SP1 or SP2)? Do you have any virus and malware scanners installed (McAfee, Ad-Aware)? Do you perform any regular maintenance (disc clean-up, disc defragmenter)? And do you have HiJackThis! installed on your machine? Have you tried starting it in safe mode?
If you have HiJackThis!, run a scan and post your log file for the scan in the HiJackThis! topic.