I want to measure the stand-by current of the PIC, but my DVM has minimum 2000micro (u) amper switch. It shows 0 micro amps when in 2000 u amps mode. Without building circuit a complex (more than 3-4 components) circuit, how can I measure nano amps?
Put a suitable resistor in series, and your DVM (in voltage mode) over the resistor.
For instance, a current of 100 nA through a 1M resistor will give you 0.1V. Check the input impedance of your DVM, for this to work it must be >> 1M. You will probably need a switch to short the resistor for the period that your target is not yet in low-power mode.
Another way would be to use a known-sized capacitor as power supply and see how fast it discharges. Again, your measurement instrument must have a high impedance, otherwise it will disturb the measurement. You might get around this by connecting the voltmeter to measure the voltage only at specific moments.
Get one of those: https://www.kickstarter.com/projects/eevblog/current-gold-precision-multimeter-current-adapter. It has also been reviewed on the EEVBlog.