can't open certain websites

everything works fine on my pc cept i can't access wincustomize.com or now sendspace.com, they both time out. haven't found any other sites yet but there prob are some. have cable modem hooked 2 a belkin router, using ZA firewall & Avast & Spywareguard, have turned them all off, diabled router firewall, even added wincustomize 2 the trusted zone in ZA & still no joy. have latest java & flash & all windows updates. using xp pro sp3. reset internet options in IE7 all 2 default, unplugged router & modem & rebooted & still nothing with these sites. have tried with IE, Maxthon2 & FF so it must be the modem. called cable tech support, all they can say is it's my pc which i would agree, but i have a 2nd pc connected 2 the router via a wireless G belkin adaptor & it can't connect 2 those sites either. i've googled til i'm blue & can't find anything that remotely helps. have run virus scan, use malewarebytes and ccleaner & they found nothing, so i'm clueless. if anyone has any ideas, i'm eternally grateful!!! Marv :cry