Some websites insist my desktop PC is a mobile device

Recently my HP Pavilllion a1510n desktop running XP (not SP3) and Explorer 7 began automatically putting me into a "mobile" website when I try to connect to www.zillow.com and a few other websites. When it happens, there's no way to make it go to the regular website. If I browse such sites with Mozilla Firefox, it doesn't happen (OK, so, yeah, just use Mozilla, I know, but I'm still curious, and it's a bother to open another browser every time this happens). The problem went away last week, but after shutdown last night it started doing it again. I connect to the web with earthlink.net via a USB WiiFi adapter communicating with a home server connected to the cable modem.