When you use the WebLogic 8.1 command installService.cmd, make sure that you first run the corresponding setDomainEnv.cmd first. If you do not, then the service will be installed without the proper classpath settings (and possibly other settings as well).
Also, there is a command-line argument -execdir:"%USERDOMAIN_HOME%". that is used by the command beasvc within installService.cmd. beasvc seems to do some non-standard processing of the command line. In particular, if %USERDOMAIN_HOME% ends with a backslash, then the closing quote does not actually close off the string. It is as if the backslash is escaping the quote. As a result, the service will be installed with an invalid working directory, one that includes all text on the command line up until the next closing quote.