There are two ways and the way to get may be OS dependent:

1.) In NT, two environment variable are set by default:
USERNAME and COMPUTERNAME
Which can be retrieved with GETENV() function

2.) SYS(0)

I like number 1 because I can use it with any OS, but for non NT machines, I have to make sure that the COMPUTERNAME variable is set on each machine and that a server login script is run to set USERNAME

I like #2 because it's simple, but I'm not sure if the result it gives is OS dependent