I have a DEM data set of Nepal and want to create a percent slope map of that data. When I use the slope function in ArcToolbox I am getting extremely high values (I know Nepal is very mountainous but i am sure that 500,000,000+ is incorrect).
Often when you perform a slope analysis and get very high values, the units are in percentages, not degrees. In theory, a slope analysis on a DEM will calculate slopes of anywhere between 0 and 90 degrees. However, with percentages, anything over a 45 degree slope will be greater than 100%. If slope is rise / run, then a 45 degree slope will have a rise of 1 and a run of 1 which equates to a 100% slope.
See the ESRI documentation on how slope works.
With this in mind, a very large rise over a very small run will give you extremely large values. This is often seen at cliffs. As Nepal is very mountainous, one should expect to see slopes like this. Try running the slope again with outputs of degrees instead of percentages.
One more thing to think about is the coordinate system of your DEM. You should be using a projected coordinate system with units in metres (or feet). Using a DEM in degrees is nonsensical.
It is hard to say what could be wrong without seeing the data. I guess, however, that the problem will be that X,Y coordinates are not in the same units as Z values. For example your X,Y coordinates could be in degrees and your Z values are in meters or something like that. Solution can be either projecting the raster and/or setting Z factor in Slope tool.