No - it is not the same thing.
Superficially, it has the same effect, but there are a few differences in what happens and what can happen if you do it either way...
In WinXP, your network connection to the internet (I'm assuming you're using a router with a firewall) has several connexion points...
The first, from the computer to the router, is through different protocols - you probably know TCP/IP (Transmission Control Protocol / Internet Protocol) - a protocol being a type of computer language.
The first disconnection occours from your computer to the router - the router, unless THAT is either unplugged or disabled, is still connected to the internet.
For example:
My Router -
I have an 2 Ethernet for PC a (running WinXP sp2, Hackintosh and UNIX) and a Mac or another PC and multiple Wireless connexions as well as USB for things like 'phones, etc...
If I were to disable the WinXP connexion by either unplugging the Ethernet cable or disabling the network through Windows Network Places - the Router would still be accessing the internet so that my PS3 and other Wireless things could get online.
The Router is still receiving and transmitting (sending data upstream and downstream) to the internet and, if your Win PC is still plugged in - there are still ways for someone to get inside your router and into your, very well protected "I'm a PC... Life without (fire)Walls...".
Your PC disconnects from the Router, but it still remembers it's IP address. This allows it to easily reconnect and be back online in seconds - it's how some people exploit the many, many, holes in Windows to "Tunnel" into your PC, through the router, and use it's - now offline - IP to "Spoof" and attempt not to get caught doing something they shouldn't...
Like using a stolen credit card, etc...
By disabling your Network Card / Adaptor through Hardware Management, you are adding an extra layer of defense.
For both your PC and the Router - there is effectively no longer a PC on the Router's Network.
You're telling WinXP not to use it - it knows it's there, but it will not allow it to be used in any way.
If you did not disable the Network Card, but just the connexion, leaving the cable plugged in - there's a major difference to disabling the Network Card and / or unplugging the PC from the Router.
Effectively isolating it entirely from the internet.
You've removed WinXP's link to the outside entirely.
If you just unplugged the cable, WinXP would tell you that a Network Connexion has been lost or disconnected.
If you disabled the Network Card, WinXP would decide it didn't even have the capacity to BE on a network...
Either way - you're off the internet and your computer cannot access it.
Both ways isolate your PC and make it a lot harder (practically impossible if there's no cable or Wireless! Or remove the Drivers or Uninstall your Network Card - but that's a pain in the neck to do every-time...) for anything to come downstream to your PC, via the Router or Internet Connexion.
Although, not to your Router...
So, even though the end result is the same thing - no network from PC, via Router, to Internet - there is a big difference in what actually happens when using WinXP.
Can you understand the difference, even though it has the same effect?
If you're worried about 'net security, you could go as far as to unplug your Router from the wall - that's still connected to the internet, even if your PC is not.
Just unpugging it means it's not online and it doesn't require a lengthy restart (even though it's only 30 seconds, it can feel like a long time...) as it would if you were to turn it off...
People can, and do, wait for a "Hanging Router" to re-establish Network Connexions to a WinPC so they can make use of it to do a number of things...
There are even some viruses that perform that task. Remaining resident in the tiny area of your Router until a connexion is made before attempting to infiltrate and exploit "I'm a PC and I use Hackintosh and UNIX and only Windows when I have to..."
(Good luck exploiting my Hack/Mac and Unix... muthafecker.)
I hate to be the person to say "No - there is a difference" when there doesn't appear to be one, but there is in terms of how it works.
In which case, just in case you're not interested the actual differences, I'll say "Yes", too...