I think it would be helpful to include the following items in your post.
(1) Your new OS.
(2) The number of physical Serial Ports you have.
(3) What other serial devices are plugged into the machine.
Com3 is usually assigned to the internal modem by default. I'm not even going to mention the Device Manager because you've obviously been there already. Beyond that issue you may want to check your BIOS to see if the serial ports that you have are activated. Doubtful though, because you would have seen an exclamation point next to the port if it wasn't. If that fails, there's always that uncomfortable excursion into the Registry.
"So much to learn. So little time to do it. Wise men know it's later than one thinks"!
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I don't answer programming questions via PMs. That's what the forum is for!