Are you sure that you really mean that it spoofs your IP address? IP address spoofing is a "black hat" hacking technique that literally alters your IP address in IP headers. But it won't work today with most ISPs, who will just drop packets with spoofed addresses.

What you probably mean is that you are using a web proxy service. Is that what you are doing?

I imagine that these very popular websites may not want people surfing anonymously, for various reasons. There are a number of ways that they could prevent people who are using a web proxy or other means to hide their IP address from logging-in.

i am using software that alters my ip address through explorer

i am using software that alters my ip address through explorer

That just plain won't work. You can't just alter your address - your packets won't get routed. You're ISP will see the wrong address, and will simply not forward the packets.

Further, reply packets have no way to get back to you. Again, the routers along the way will either simply drop reply packets (this is assuming that your outbound packets ever got anywhere - which is a BIG "if"!) or will route them to the real owner of the IP address.

This can work under some very constrained scenarios. For example, you MIGHT spoof your address at work to make it appear you are the fellow in the next cubicle. :)

But with a typical dial-up/DSL/cable modem sceario, either your router or the next one up the line is just going to drop the packets.

oh by that i just mean that it is going through a proxy.

It is possible that those sites are simply blocking access from a number of proxies, and your proxy is included. This is not uncommon.