Install jbpm-4.3.zip by unzipping to C:\ and perform manual install on Tomcat. The get.tomcat task in the Ant install/build.xml points to 6.0.20 version (line 31: <property name="tomcat.version" value="6.0.20" />) and tries to get that version from apache.org. The problem is that this version, 6.0.20 is currently not available at apache.org, only 6.0.24 and 6.0.26 are available.
Fix: Modify install/build.xml in order to get newer tomcat version or point to another location that holds 6.0.20 version.
Workaround 1: Place manually apache-tomcat-6.0.20.zip in /install/downloads getting it manually from another location
Workaround 2: Modify install/build.xml and put 6.0.24 or 6.0.26 version in line 31, but this version could not be supported for jBPM 4.3