Internet speed nosedives on all networked machines once my Ubuntu PC boots and joins network

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http://askubuntu.com – My home network: Cable Modem : WebSTAR DPC2100R2 Wireless router: Netgear WNR1000 PC1 (winxp) : 3G ram, 2.6GHz Intel, Asus 5PB Wifi delxe, running Windows XP SP3 and connecting wirelessly to router/internet. PC2 (linux) : 1G ram, 1GHz AMD Athlon CPU, 3Com 3c905 100BaseTX Nic, connected by ethernet cable to router, onto which i have just installed lubuntu 12.10. With PC2(linux) disconnected from router, PC1 (and all other wifi networked devices, for that matter, though for the sake of diagnosing this problem, i've powered all other devices off), PC1 scores a download spee (HowTos)