Within Windows you can either crank down the NIC's speed by configuring the advanced properties of the network connection (although this really just lets you set a 10/100 card at 10) or you can twiddle with the registry. All the registry settings are listed in KB article 314053 at:

http://support.microsoft.com/default...&Product=winxp

I've fiddled with this in a few cases with so-so results. I'd really recommend one of the options already listed, particularly phishphreek80's idea.