At least on XP and 98, having a printer defined in windows that
uses a com1 or com2 etc port for output makes most other
programs unable to use it, even if you never ever print to that printer.
Echo ath1 > com1 at a windows command prompt will still work,
and other printers can still specify that port, but the windows applet
Hyperterm, and [more importantly] a d3nt session trying to do a
DEV-MAKE on that port will fail.

If this happens, free the port by clicking the PORTS tab in that printer's
properties and check a different port, or make a new port c:\foo.txt and
check that box. If you later want to put it back as you found it, it seems
to remember the port settings if you just re-check the com1 port's box...
Repeat for all printers using that port. In w2k and later you may not have
permission to write to c:\ directory, so if c:\pub exists use
c:\pub\foo.txt
Port FILE: is a little different, it pops up a requestor at each print
job
asking for filename to 'print' to -- very annoying in some circumstances.

If this doesn't help, you may have some process using the port; google
just advises 'search the registry for that com port' for this situation.