It would only invalidate that claim, but barring finding other prior art for the other claims, the rest of the patent would stand.

ww520 above asserts: A claim is just a list of "AND" conditions, i.e. all the conditions must be met for the claim to be asserted. To defeat a patent claim, it's a matter of adding enough variance in your code to defeat ONE of the conditions.

One of you is right. Which one?

A patent can have many claims.

For instance,

1. A system for sending and receiving email.

You can probably find prior art for that.

2. Claim 1 where the email is sent over a TCP/IP network

And probably you can find prior art for that.

3. Claim 2 where the network is wireless

NTP won over $1 Billion for filing a patent with a claim about this serious, mostly from Blackberry.

Now you can continue like this indefinitely. That's why every time there's a new system, there are thousands of new patents that can be issued. Email on a phone, email over multipath TCP, email with a touchscreen, email with MIME messages, email with MIME messages on a touchscreen, &c. Each of those is considered valid to patent by the PTO. Then instead of email, try playing MP3s, or word processing, or online backup. You quickly arrive at a thicket of a million valid software patents covering everything you might ever want to program many times over.