Computer reboots when I go online

I have a pentium II 350 processor and 64 mb of ram. Using Windows 98 SE, Email program using Outlook 2000, I connect using dialup, and stay resistant programs are norton antivirus. I also have an emulation card to access the AS/400 system. I havealready replaced the 1st internal modem with a Zoom 56k PCI modem and still have the same problem of my computer rebooting when I go online and retreive my email (occasionally). This only started happening when I got this new / faster computer