I was have same problem with my usb port.
You can change default com port number under XP:
Go hardware admin, select your com11 port and go "properties" button. goto "config port", select a com2 (or a free com port)
If all ports below com9 are "in use", select a sure free com port (for example com5) and agree warning.
My windows isn't english, some text may vary. screen capture attached.
hope help.
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Mendel Prusa with Gen7 v1.2 PCB , Wade's extruder 0.7mm. Custom Heated bed.
"BIG" Mendel Pusa with modified Gen7 V1.3 PCB , Wade's extruder 0.5mm with modified hotend (about 500x600x450mm printable area) <--- under construction
Plastics: Black PLA, Soft PLA, White ABS
Software: Skeinforge 4 for STL to GCODE, RepRap Host for print GCODE. Blender for STL designs.
