Bo Jingly,
I can assure you that your Router and Modem are not the cause. There is something else going on in your computer. Try running something that is very demanding on your computer but does not use the Internet. Lets see if that causes it to reboot also.
You could try something like
3DMark2001SE Build 330. (Use the
Free Version)
Lets see if that reboots the comp or not. If you can get though that benchmark without problems, lets look at your NIC.
It would help if you listed your actual hardware. (Mobo, NIC, Vid Card, HD, etc.)
Take Care,
Richard