Network card

I have a Verizon Quantum FiOS Gigabit Internet connection. My Toshiba laptop's Internet speeds exceed 900Mb/s. Here's my question: I got the old tower computer I built years ago networked directly to the router by Ethernet CAT5e cable, yet I can only approach 100Mb/s with the overhead. It has an AMD Athelon 64 3000+ processor running at 1.81 GHz with 2GB of RAM The OS is Windows XP Pro 32-bit (sp3) Version 2002 and the motherboard is an ABIT FATAL1TYAN8 with Nvidia onboard networking. I disabled it in the BIOS and got a TP-LINK TG-3269 Gigabit PCI network adapter and adjusted the properties in Device Manager for 1000Mb/s, yet I am still limited to 100Mb/s. Is it possible that this computer just can't handle Gigabit speeds? Any advice would be most appreciated. :)