sorry, i should have been clearer.
if you are going to attach one of the easy radio modules to a PCs (RS232) serial port you will need to use a MAX232 level shifter (or something similar) to convert between the RS232 voltages on the PCs port and the TTL levels on the easy radio module.
it's a fairly simple circuit at the PC end. there is an example of how to do this on the easy radio datasheet.
USB is quite a lot more complicated.
i'm only learning about interfacing to USB devices so can't really be much help here but as a basic overview:
USB equipment is separated into "HOST" and "DEVICE" categories.
the HOSTs controll the USB connection and require a lot of programming. you only really find HOST ports on full-blown computers.
the DEVICEs are anything you can attach to a USB HOST port. these require far less programming and hardware. there are microcontrollers out there with USB DEVICE ports built in. all the USB equipment you plug into your PC (mice, keyboards, webcams, memory sticks, etc) are USB DEVICEs.
there are 2 common USB standards in use on modern computing equipment. USB1.1 and USB2.0.
USB1.1 is an older, low speed implementation.
USB2.0 is newer, high speed.
because USB is backwards compatable you can plug a 1.1 or 2.0 DEVICE into a 2.0 HOST port.
all new PCs ship with 2.0 HOST ports so unless you have a really old computer on your desk, chances are you have USB2.0 on it.
most low speed USB peripherals (like keyboards and mice) use USB1.0 as it's cheaper to manufacture.
things that require high data through put (like webcams and USB hard disks) will use USB2.0.
it is possible for the hobbyist to build USB1.1 devices at home.
here's a very good starting point: http://www.obdev.at/products/avrusb/index.html
i haven't seen anyone out there who has successfully built USB2.0 DEVICEs or any sort of HOST without access to a lab that can produce surface mount boards and a huge amount of work.