Xbox 360 Wireless Adaptor

You can build one from an old RROD 360. There are many guides, but the simplest method is just adding 2 diodes and editing the USB ID in the driver. You can get more complex using an Arduino, but the basic one has worked for me for years.

http://dilisilib.wordpress.com/2011/03/07/wireless-xbox360-controller-on-a-pc-without-the-commercial-dongle/

You can sync by plugging it into a 360, or using a USB charging cable also plugged into your PC to connect the 360 controller. Once synced, it'll always work.

Otherwise, you can just buy something like this for around $10:

http://www.dx.com/p/pc-wireless-gaming-receiver-for-xbox-360-controller-black-129230