Can I run a 12v fan off Arduino or sensor shield?

Hello, this is my first post here so please be gentle with me. I'm a C programmer so I know my way around the code, but an utter electronics novice.

I've started a little project to monitor temperature and activate a servo at a given threshold (its for a PC in a cupboard and it opens the door a crack when it warms up). I got that working and was all pleased with myself, but now I'm struggling with something that is probably a bit of a no brainer.

I've searched these forums but cannot find what I need. Now I want to start a fan when the temperature reaches a slightly higher temperature threshold. I don't want to control the fan speed, I just want to give it full beans until the temperature reduces.

I have my Arduino connected to a USB port on the PC in the cupboard, so this is how I'd like to power it forever more, i.e. without using any other external transformer. I have a sensor shield v4 attached, the temp sensor on pin 3 and the servo on pin 9.

I'd really like to use a 12v PC fan. Can I run this at 5v and will it just spin much slower? And can I attach the three pin fan connector to one of the digital pins on the sensor shield?

Apologies if this is a stupid question, but any help would be greatly appreciated. Thanks very much.

The fan will probably rotate on 5V, but digital out almost certainly can't handle the current the fan would draw, at least not when the fan starts up. If Arduino is powered from USB, you can draw a few hundred mAs from Arduino's 5V pin, if that's enough for the fan. You need some additional electronics, e.g., a transistor and resistor to switch the power to the fan.

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1273096221/15

Thanks very much for taking the time to reply. I'm just recalling my previous attempts at soldering :-[, so I reckon I might just stick with cracking the door open to reduce the temp! Or I might try to get hold of a 5v fan. I'll have to see how brave I feel...

Cheers fellas.

Treat the fan just like a motor:-
http://www.thebox.myzen.co.uk/Workshop/Motors_1.html