http://www.arduino.cc/playground/Learning/Tutorial01
Now if you can get a USB bluetooth dongle, shouldn't you be able to use one of those to communicate with the USB dongle on the computer?
Drivers could potentially be an issue, but there should be a way!
Is there?
Also, can the arduino convert A/D signals?
The ATmega168 has a 6channel ( 6 pin ) 10bit AD converter
Now if you can get a USB bluetooth dongle, shouldn't you be able to use one of those to communicate with the USB dongle on the computer?
The "USB Dongle" for your computer is a USB "device", and the arduino can't talk to it. (in general, one USB device (arduino) can not talk to another USB device (bluetooth radio.)) The USB dongle is probably also much closer to a "bare radio" than the "modem" mentioned in the playground article, and is probably controlled by more software in the PC-side driver than would FIT in an arduino...
Many USB devices designed for PCs offload a lot of the work to the host PC system ( using the PC's CPU ) to save on cost, that way the device does not need powerful and expensive ICs of its own
Usually the more intelligence and power you put into a device the more it costs
As westfw mentioned the Arduino cannot act as a USB host so you cannot connect USB devices to it
Even if you could, the arduino would have a hard time running the driver software if any actually existed
It may be possible to build a USB Host shield for it, but even then you would only be able to talk to a limited number of devices due to driver limitations, RAM and CPU power and storage requirements
Remember the ATmega168 ( 8bit MCU ) has 1K RAM, 16K flash and usually runs at 16MHz
A typical PC has 1GB RAM, 200GB HDD space and a CPU ( 32 or 64bit ) running at 2+GHz with 1, 2, 3 or 4 CPU cores