I'm going to be buying a laptop soon. Its VITAL that I have a 9 pin serial port. I know new laptops don't have these, so I will have to use an adapter. I've heard these adapters are "iffy", in that they sometimes work and sometimes do not. My question is which operating system is more likely to recognize a serial cable plugged into a USB adapter, XP Pro or Vista?

I personally have used an USB to Serial cable adapter on just such a laptop under XP pro at my last job for RS-232 interfacing. So I can vouch that with hat particular adapter at least, Xp Pro is very reliable.

Unfortunately I can't remember which brand we used, but it was a pretty generic low-end one in any case [read: cheap], so I'd look around Newegg. I think it was a Rosewill, one of the cabley ones not the boxy ones.

Serial-to-USB adapters/dongles are very easy to find.

Both are equally likely to work and you should be able to find a dongle that has signed Vista drivers too. Ideally, with the drivers installed and the device connected, it should appear as a serial port for I/O.

Some of our instructor stations @ school need a 3rd COM port so we installed USB -> RS-232 adapters. Only one has malfunction but that was because someone screwed up the driver install.

We use a USB to 15 Pin Serial Port adapter for robotics club and it works all the time (the software team tells me that NOBODY makes computers with 15 pin serial ports anymore)