You could get a PCI USB card and plug it into that, VIA chipsets(at least the earlier ones) have notorious USB problems especially with ADSL modems.
The PCI cards are so cheap now it wouldn't hurt to give it a try even if it doesn't work you get a few more USB ports.
Also bring your task manager up and watch the cpu usage when you open a divx clip does it go up to a 100% ? even briefly and does it disconnect at that moment.
You could try a updated driver but it appears wisecom usa + europe no longer exsist according to the links google provides.
Wisecom dont make the actual driver anyway it's Globespan but you would have to find an identical device that uses that chipset and/or a generic driver from Globespan.
Their is a driver that is labelled compatabile with WinXP at this link give it a try and lets us know.
Wisecom USB Driver
Also the driver uses this desciptive term below (the Annex bit) so match it to the service you have.
Annex-A ATM = PPPOA
Annex-A LAN = RFC1348 Bridged Mode (use with RASPPPOE)
Annex-A WAN = PPPOE