It's not possible built into XP, and even if it were, you
would not see any performance gain at all. Before I go
into the details, what exactly is it you're trying to
do? Get a faster Internet connection?
Why? Because it's not the network card that slows you
down, it's the Internet connection. Network cards run at
at least 10 Mbps, and can support full duplex (both
incoming/outgoing). Most cable ISP's cap your connection
at either 3 Mbps or 1.5 Mbps down, and only 128 Kbps or
256 Kbps up.
In short, your network card can transfer much faster than
your Internet connection can. Adding another network
card won't do anything.
There are hardware devices that support what is known as
Bonding, where you have two Internet connections (ie,
multiple cable modems) and the hardware piece does some
work to make it seem like one faster connection to your
computer. These are not cheap, and generally aren't
worth the trouble unless you really feel like spending
lots and lots of money for multiple dedicated Internet
connections, etc.