First, If you have a Broadband modem, especially an Alcatel, you should upgrade your drivers.(For other drivers go here)
Linksys network card Drivers If you have a Linksys NC100, LNE100TX model 5, Skymaster SK1207E, Planex FNW-9803-T, or any network card based on the AN983B chipset by ADMtek, also sold under Asound or Fast, motherboards' built-in Ethernet controllers, such as: MSI (Microstar) MS-6378, DFI NS70-EL & AZ30-EL, USI PM-845, Fujitsu D1451.. Update the drivers
Netgear FA311 and Netgear FA312 are also known to have issues. Try this driver update to resolve.
This is a NIC (Network Interface Card aka Network Card) related problem. most problems associated with NIC's - especially when running on 2000 / XP - are interrupt based.
Win 2000/XP has special problems because Microsoft's shortsighted default configuration assigns as many devices as it can to a single interrupt. Your computer then 'load balances' the lot. Don't ask why, its Microsoft.
Load balance depends on the Operating System to find out which device is asking for service and answers that device. This is all fine and dandy as long as you only play minesweeper and solitaire, but if you use things that actually use a lot of resources, this can cause real headaches.
The solution, for a number of people's problems, it to make sure Windows is not IRQ (Interrupt Request) sharing anything else important with the NIC. Some cards are a bit choosy about what IRQ level they use, but I have had no problems EVER, when using IRQ 9, 10 or 11 as long as drivers are current, and there is no sharing conflict.
First verify that your NIC is IRQ (Interrupt Request) sharing. You can do so by going into the device manager (control Panel, system, Hardware tab, and there should be a button called device manager about halfway down).
Once you have Device Manager open, select VIEW from the toolbar menu and then click Resources by Type.