:\Program Files\Java>ant
Unable to locate tools.jar. Expected to find it in C:\Program Files\Java\jre1.5.
0_10\lib\tools.jar
Buildfile: build.xml does not exist!
Build failed


It is looking for tools.jar in %JAVA_HOME%\lib. JAVA_HOME should point 
to j2se SDK installation. In your case, this is j2se RE - which does not 
have tools.jar.