IDA-Pro embedded debugger doesn’t support PE files with zero image base.the debugger says (I quote):“IDA Pro couldn’t automatically determine if the program should be rebased in the database because the database format is too old and doesn’t contain enough information. Create a new database if you want automated rebasing to work properly. Notice you can [...]

http://souriz.wordpress.com/2008/05/14/773-bug-in-ida-pro-fails-to-debug-zero-base-pe/