well, you can't connect stuff directly to usb. every smart usb device (that communicates with a computer, usb lamps, fans and these toys excluded - unless they have software to control them) has a microcontroller in it. this thing does the communication and interprets its sensors. so if you want to read your accelerometer, you can't simply wire it to usb and expect it to work, unless it has an usb interface (but that's a completely different thing).
think about it, usb is designed to handle 127 devices in parallel. if you connect to accelerometers and they send different values, how would your pc know which value to pick? it won't be seeing a correct value, since the two voltages will interact with eachother.
the serial port is by excellence a digital port, so you can't feed it an analog value. you might have some luck with a gameport, if you want a direct analog input, however, you will need to write specialised software on the computer side.