For a desktop, I would recommend a PCI Wireless Adaptor, but a USB Wireless Adaptor (one which plug in the back of your PC) will be just fine.

Windows may be able to detect the Device and then it should be able to let you connect to the network/internet. If this is not the case then you will need the drivers, which would be in the manufacturer's website or a CD that came with the device.
Once installed it should find the signal just fine

On the note that you are chucking out your iMac: DON'T.
I don't even know why you would want to they are more secure than a Windows PC, when it come to the internet.