Weird serial mouse problem on WinXP

I've just installed winXP SP2 on my syster's computer and during the setup her serial mouse worked just fine but after the setup was done it simply stopped working. The weirdest part os that you can move the mouse pointer on the "Welcome to Windows XP" screen (right after boot finishes) but when the desktop appears the mouse stops working again =/ I've already checked for conflicts or driver errors on the device manager but everything seems fine, what should I do?