USB will need a lot more powerful PC than a dedicated ethernet. If I recall correctly spec is quite low. I could run broadband on my ancient P200MMX that only had 64MB RAM and win 98se, with a network card. Didn't do it very well though.
Edit: Beaten to it with a more comprehensive reply

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If the PC has fallen over, it's quite possible that it didn't have adequate antivirus or something, and once connected that has swiftly picked something up. Also there is a possibility that it was previously doing dial up, and the dial up settings now conflict with the broadband.