So it doesn't necessarily mean the site has been hacked into, but it's more like a representation or proof of whoever is behind that particular secure connection that you're about to provide security details to?
Should I be worried that there will be malware downloaded on to my computer too? Sorry I get easily worked up over these things. I'm in a rush too, gotta go out soon, so I haven't read the wiki link yet :(

Thanks much for the update. :)

I'd personally say don't be worried at all. if you have any concerns, then don't enter your payment information, but it's not going to make your computer less safe or anything. All this says is that the site is trying to initiate a secure connection (a good thing), but that the site that is trying to do it is giving a bad certificate (a bad thing).
Apparently in this case the regular cert got replaced by a bad cert- it happens. If you see it anytime in the future, if you're going to give payment information to the site in question, I'd recommend not. If the payment information is going somewhere else (i.e. paypal) that will initiate it's own secure connection, then there's no way that they can get the information from the other secure site (paypal doesn't transmit your payment information to them- just something to let them know you paid).

Not while already logged in.
Fishing up login information as it's being typed in / transmitted to the server is easier, but would at the very least require a running script on the offended computer to intercept the information. Such scripts can be embedded in bad ActiveX-controls for example.
That or a keylogger software running in the background.
As long as you excercise the minimum of caution you should while hopping about the internet you should be safe.

Ah ok that's a consolation. Thanks.