If you're using TCP/IP as your only
network protocol, a quick & dirty way do it in XP would be to have the NIC drop it's IP address assuming you're using DHCP to get your IP address from your router.
This doesn't disable the NIC per sce. But it does render it unable to access the network, so it effectively kills any chance of connecting to it.
To "disable" use: ipconfig /release
example: ipconfig /release "Local Area Connection"
To "enable" use: ipconfig /renew
If you only have one NIC the adapter name is optional.
For hotkeying, you could write each command string to a separate batch file (e.g. killNIC.bat and wakeNIC.bat) and invoke them with whatever hotkey mapping utility you'd prefer. There are dozens of free ones available for download. You could also map them to an unused function key if your keyboard comes with keymap utility software.