Comment 2 for bug 438141

sounds similar to Bug 371386, particularly comment 5.
could you check your preferences file for the start and end values of the arc? (they should be 0)
On Windows this is at :
C:\Documents and Settings\username\Application Data\Inkscape\preferences.xml
- alternatively, you could try renaming this file to be something else, which will force Inkscape to use fresh defaults, and may solve the problem.