Setup COM1 on XP guest

Assuming that you called the serial port COM1 in the VBox setup and that you also selected Host device. (This is what it needs to be by the way)
You then start the guest (XP)
Windows will not see this and there is no way that you can make it see this port as plug-n-Play but you can make it work.
1. Go to control panel (select classic view) and double click new hardware.
2. Click next to start the discovery process.
3. Select yes I have already connected the hardware then click next.
4. Scroll to the end of the list and select add new hardware device and click next.
5. Select Search for and install the hardware automatically.
6. When the scan has completed you should see the Com port.
7. Select finish to end the process.
8. Open the device manager (right click my computer and left click on manage) select device manager.
9. Click the + beside ports and you should see the com port.

Take a bow, Tip of the hat, go on with your work.