slow internet speed

XP, Cable modem by router home cabling (in walls), says she opened an attachment and a day later she cannot get on the internet, go to her e-mail, some icons on her desktop have been changed. I ran systematic anti-virus and found nothing, I checked her speed it is down to almost nothing. I connected another computer to the same line (I disconnected her and connected another to the line that was on hers) and it worked perfectly fine fast internet and all. Any help?