Spurious Garbage printing on my serial printer from XP

I have a strange problem with a number of PCs I have recently
deployed. The configuration is as follows :-

Fujitsu Siemens E2500 running XP pro. An Epson TM88 serial thermal
printer connected to COM1.

On two of these machines at one particular site, the attached printers
start printing garbage as soon as XP Pro is fully launched, logged in
or at the log in prompt.

The problem follows the PC as I have swapped printers and other
devices to make sure.

If I disable the COM1 port is stops. If I renumber the port, it still
prints garbage. The machines are brand new and do not have any spyware
of viruses on them.

I did a bit of investigation and found that another client of our
sister company had the same problem. More interestingly, they at first
thought it was the printer and took it home to try on a PC they had.
This machine was not a Fujitsu but was a dual boot 98 / XP Pro unit.
The problem only presented itself on this machine when using XP. In 98
it was fine! I am now at my wits end and have a potentially difficult
situation with the client as all they want is for it not to be doing
it. They do not care what is causing it. Anyone out there seen this?
How do I fix it?