You need more than a sim slot, you also need the 3G modem in your machine as well as the software for it.
But if it's a 3G unlocked modem (or one locked to AT&T) and it supports the AT&T 3G frequencies, you'll be able to surf for free on your PC. BUT, since the iPhone plan is officially only for smartphones, AT&T might decide to bill you for the usage. A lot of people do this, and if you do it in moderation, you do indeed get free unlimited data.
You'll need to google for the AT&T 3G settings.