I'd be very surprised if its the modem itself causing the problem... if i were to hazard a guess, i'd suspect spware, adware, virus or network stack issue of some kind
there's plenty of free applications to check for spyware:
adaware
spybot
MS antispyware
MS malicous software removal tool
and recently have found the ewido security suite to be great...its a paid for application...but is free for 14 days, with full functionality.
info here on win xp tcp/ip refresh
http://www.chetnet.co.uk/articles/in...x_v2&id=35&c=5