What kind of network are you on? Wireless B,G,N? Wired? What speed is it?
Do you have a router? Wireless or Wired or both? How do you connect to the internet?
Cable, DSL or other? Does your computer use static IP or DHCP to get the IP address?
Also include info. about both PC's
The problem you describe could be caused by several different things depending on those questions.