I used to be a Psion enthusiast and my Psion-based website is still in place, although it hasn't been updated for a number of years now. PsiWin connections were always somewhat temperamental and they frequently didn't work with USB/Serial adapter cables. If you want to use a Serial COM port, I'd strongly recommend installing a PCI Serial Port card to implement the port properly. The question that I don't know the answer to is whether PsiWin 2.3.3 will actually run in Windows 7, if that's the operating system you're going to be installing.
You don't mention which model of Psion you have but, with a Series 5 and 5mx, I never actually connected them with PsiWin. Instead, any files I wanted to transfer to the PC I put on the Compact Flash card and then plugged that into a CF reader in the PC. My internal drive was copied to a folder in the CF card so that the entire Psion was backed up in the PC.
Example of PCI Serial Port Card