Yes, your Java runtime is too old, according to Java class file format:
- J2SE 7 = 51 (0x33 hex),
- J2SE 6.0 = 50 (0x32 hex),
- J2SE 5.0 = 49 (0x31 hex),
- JDK 1.4 = 48 (0x30 hex),
- JDK 1.3 = 47 (0x2F hex),
- JDK 1.2 = 46 (0x2E hex),
- JDK 1.1 = 45 (0x2D hex).
51.0 means you need Java 7 to run some of the classes in your project. And you are right it's jTDS that's causing the problem (from jTDS JDBC Driver 1.2.7 and 1.3.0 released):
Version 1.3.0 is the first Java 7 compatible version of the driver and
Either upgrade to Java 7 (always a good idea) or downgrade to some older jTDS driver.