I found an intresting one today. the store was getting turn on you modem please in 6 series polling. the modem was deteched in windows and we could query the modem in windows it would respond fine. so we knew that the modem was working. Went to a DOS prompt and typed in MODE COM1 and it returned the COM PORT IS NOT AVAILABLE.

found on line that there is a problem with the port starting correctly, (althought it still worked in windows)
Right click on the port in device manager disable it and then just renable it. that fixes it. we tested our theory by rebooting the PC and again it wouldnt work, disabled and re-enabled worked fine.

they were on an older windows 2000 SP3
sending the SP4 to see if that fixes.

In answer book