Slow LAN File Transfers (and more frustrating network problems inside)

I'm totally confused. I'm normally able to eventually figure these things out, but I'm just stumped on this one Here's the scoop:

Just recently built a new PC (running on WinXP Pro). It's got an Abit AV8 motherboard which has a built in 10/100 network adapter. Here's the fun part:
Lets say I transfer a file from the new PC to my old computer. Everything's fine. Now, lets say I take that same file and try to transfer it back (from the old machine to the new). It takes about 10x as long. 16mb file clocking in at an ETA of 5mins. ????

So, I tried transferring to other machines, tried setting it to forced full duplex, 100mbit, etc. No go. Drivers have been reinstalled multiple times. I've even turned it off in BIOS, restarted and turned it back on just to see if that would do anything Nothing, nada, no go.

And also, which I haven't taken the time to confirm 100% because I'm just frustrated with it, I believe that the same thing is happening with downloads as well. I thought it was just a slow server at first, til tried multiple sites. It starts off at my norm, 80kbps or so, but immediately starts to drop and hang out at like 10-15kbps. But every single other machine on my network is fine. It's just that one.

Any suggestions?

It came with SP2 =\

Sounds like it's a hardware problem. Just found out from my dad when he temporarily had Win2k3 on there he was having the same issue

Time to swap in a NIC from my other box and see how that goes