Try "COM1" without the quotes. That is what you would commonly use on the desktop and is the example given when you hover over a VISA refnum or terminal.
That is actually an alias for "ASRL1::INSTR", which is the full name that VISA uses to describe the serial port.
For more documentation, here is a VISA manual.
http://www.ni.com/pdf/manuals/370423a.pdf
Serial ports addresses are on page 18 of chapter 9.
Greg McKaskle