Ok, you asked for my version of the circuit.




This uses an opamp+BJT current source with a three-decade range.  The range of the current source is selected by grounding one of three resistors.  You can probably achieve your accuracy goals by using AVR outputs to switch the three resistors.  Switch between output low (for enable) or input (for disable).  Analog input is better, but the voltage will be an unambigious high, so digital input is OK.  For better accuracy, connect the 4K to resistor to two pins.  The output resistance of an AVR digital out is about 25 ohms: 

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The +5V line is used for the reference of both the current source and ADC.  Variations in supply voltage will cancel.  The alternative would be to have a reference in the current source and a reference in the ADC... not necessary here.  Microcontroller ADCs are generally happy to use the supply rails as reference.
You must make four connections to the device under test.  Two of the connections deliver the current, and two of the connections present the voltage across the device under test to the measurement circuit.  Four-wire connection is necessary to measure low resistances ( &lt; 1 ohm )!  Otherwise you are measuring your probe resistance by accident.
The opamp's offset voltage is the most important parameter.  Use a chopper amp and don't worry about it.  I've spec'd OPA2333, which is a nice slow amplifier that's always worked well for me.
If your probe resistance is higher than about an ohm, you should go for the full instrumentation amplifier.  But with reasonable probes this should meet spec as-is.


Measuring Resistance of a Wire With an ADC
I have a Windows XP product key on my PC, but I'm not sure which release of Windows XP it belongs to SP1, SP2, or SP3.

Is there a way I can tell without trying each one?

Note: 
In this case, I have an old PC which had an OEM XP Pro license. I don't have the media for it any more. However, I do have a media for XP Pro (off the shelf non-OEM). 

The Product Key on the case doesn't want to validate. So I'm presuming that there is something encoded in it as to the release it works with. I can get media, I just don't want to try each one.
