What is your output when you do java -version? This will tell you what version the running JVM is.
The Unsupported major.minor version 51.0 error could mean:
Either way, uninstall all JVM runtimes including JDK and download latest and re-install. That should fix any Unsupported major.minor error as you will have the lastest JRE and JDK (Maybe even newer then the one used to compile the Servlet)
See: http://www.java.com/en/download/manual.jsp (7 Update 25 )
and here: http://www.oracle.com/technetwork/java/javase/downloads/index.html (Java Platform (JDK) 7u25)
for the latest version of the JRE and JDK respectively.
EDIT:
Most likely your code was written in Java7 however maybe it was done using Java7update4 and your system is running Java7update3. Thus they both are effectively the same major version but the minor versions differ. Only the larger minor version is backward compatible with the lower minor version.
Edit 2 : If you have more than one jdk installed on your pc. you should check that Apache Tomcat is using the same one (jre) you are compiling your programs with. If you installed a new jdk after installing apache it normally won't select the new version.