download intsall update and scan your pc in safe mode with ewido anti-malware
www.ewido.net
also use 'ccleaner' to clear ur pc junk.
www.ccleaner.com
if it dznt help, post ur hijackthis logfile here or at
www.hijackthis.de for analysis.
this is to isolate and treat ur problem' once addressed, u can install precautions like za firewall, etc.