...Ahh, Dell Laptops! I have experienced similar problems with Dell laptops. It would seem that the current applied to USB sockets is variable/unstable in some cases. In your case, 440 ma, I think, would be difficult to achieve, let alone sustain.
I think RS232 requires about 400ma. I'm sure you now understand the problem a bit better - it probably will be difficult to gat your device working! But, who knows, you may get more positive help from someone else.