System Calls and Variables?

What is the syntax in C# and where might I look to locate System variables? This is actually two questions.... Sorry in advance. In essence, can I call a Windows OS system variable to determine who the current user is? I'm trying to set a path for a file to be written and I want the file to go into the logged in user's "Document and Settings" folder. Lastly, what System function is used to call an operating system function? If I can get some pointers to locate this in the documentation, that would be good too... Thanks.