Hi Alexander,
Firstly: This error generally refers to the 'class path' variable not being defined or it is incorrectly defined. When the Java program starts to run it uses the operating systems variable called 'classpath' to locate the libraries/classes that are delivered by the JVM.
Go back over the java installation instructions and ensure the classpath variable is defined in your operating system, properly.
If that did not work then ...
Secondly: The version of the java software your are running is generally supposed to always be forward compatible. In theory you should be able to download the latest version of the JVM and use it to run older java programs. Unfortunately the SCN forum is full of examples where it is not this simple. Conclusion, locate the SAP Netweaver 7.01 installation instructions, it will state the exact version of the JVM that it was written for, then go the older download section for the Java software and install it on your system to be run along side the SAP Netweaver 7.01 software.
Hope this helps.
Kind regards,
John.