Cannot access internet with Vista HP

Have a new Dell studio machine with Vista Home premium. Connected via Linksys BEFSR41 router to cable modem. Using Antivir for antivirus, Eudora for email client, Office 2k, software for HP 4360 Deskjet printer. That is the only software installed that did not come with the machine. Intermittently I cannot access internet using IE7 on this machine, while a second desktop running Win2k through the same router has no problem. Dell insists it's a software problem, suggested using Firefox. This is making me crazy. Has anyone else had this problem or a solution?