Vexations of connectivity

Yesterday's Internet problems were worse today - at one point, I couldn't even open a small file on the office network drive (via vpn). After about three hours of struggle, including using AT&T's speedtest repeatedly, I disconnected D's ethernet cable from the router, and suddenly our speed increased by a factor of 300. Unfortunately, connecting D's computer to the wireless network made everyone's connectivity go bad again, although not as bad as before. We ran a bunch of antivirus scans on it, and cleaned up some malware, but things are still suboptimal, and I have lost patience with it for today.