When i said missmatching drivers i was thinking that the drivers for the ps2 mouse were too new for the operating system or they were designed to "see" the mouse in a normal ps2 port, not an old serial com port.
Another idea, you could try to scrounge up some old mouse drivers that were specifically designed for serial mice.
And yes, downgrade is bad.
What OS are you running?
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Not all PS/2 mice work with any adapter. The ones that do work with an adapter might not work with the one you have, as there are several configurations of adapter. I remember for certain that Logitec used a different one that Mitsumi, for example.
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The difference between PS/2 and UART (normal com port) is not just an adaptor. PS/2 uses a different signaling, so in order for your adaptor to work, the mouse must be capable of that signaling. Some mice only supports UART signaling, and those cannot work through a PS/2 connector regardless of drivers and adapters.