Pulling the network lead (or disabling wireless) is sound advice and is good practice. You should do this the very moment you are aware that a PC has become infected?
Why?
1. To stop malware from infecting further PC's on your network and beyond.
2. To prevent further infection of the PC (often a malware attack can make an infected PC vulnerable to further infection by creating backdoors and compromising your anti-malware software).
3. Often the first attack is just the infection whereby the malware then 'calls home' to notify the writer / author that your PC is primed for whatever deed they have planned for it. This could be reporting your keystrokes and website visits via a key logging script, uploading your files such as browsing history, or using your PC as a 'zombie' to attack other PC's and networks.
4. In many cases you may find that your anti-malware software is unable to complete an update as one of the first things the malware might do is to try and prevent it from getting updates (I've seen this in a high percentage of infections). 'Good' or 'effective' malware attempts to attack the lower level operation of your PC (the area of the operating system that controls your hardware and networking). By using methods such as DNS changing, you will have no idea whether the websites you are visiting are genuine or are infected mirrors.
As well as downloading the program from another machine, many anti-malware programs provide a way to also copy updated definitions from another computer.
Here's the recommended method for using a clean PC to update the MalwareBytes definitions of an infected PC:
http://forums.malwarebytes.org/index.php?showtopic=10138
(See point 4.).
