Hmmm... I have MoH installed on my Dells and COM1 (9-pin port) works fine. Modem is COM3; Ir port is COM2.
Have you loaded the chipset drivers since the last install of XP Pro, bobnijip?
If you go to
http://support.dell.com/support/downloads/index.aspx?c=us&cs=04&l=en&s=bsdand enter either the Service Tag or select the proper model from the picklists you can get the latest versions if you can't find the Drivers and Utilities CD.