Using an iPod charging cable to charge an iPad I lost the original plug-in charger that came with my iPad.  I have tried using my iPod cable to charge my iPad (plugged into the USB port of my Windows laptop) but the iPad will not charge (or at least it charges extremely slowly).  I also have tried using another USB plug adapter with the iPod cable (plugged into a wall outlet) and it also does not charge.  What is going on here and how can I charge my iPad now that I have lost the original cable and outlet adapter that it came with?
The iPad uses a 10W charger.  The iPhone has a 5W charger.  Most older computers only support the original USB spec of 2.5W USB port power support.  Your iPad, when on, uses more than 2.5W, so it will not charge unless it's turned off, and when off it will charge at less than 1/4 the speed of the normal iPad charger.  That can be up to 16 hours for a fully depleted iPad.

Newer Apple computers, and computers that support USB3.0, support higher USB power standards, but if your computer doesn't then you'll need to invest in a new charger for your iPad.

Note that the iPad will generally charge off other USB chargers from other manufacturers, but only at a slower rate than the original iPad charger, unless the charger you use is specified to charge the iPad.  When the original iPad came out 10W USB chargers were not common.