Quo vardis is right the key he's trying to use is corrupt imo.
@ Snattix make sure you can see hidden files and extesnions in windows 7 (use google to see how, if you haven't done it already)
ET key is a hiddden file and has no extesnion to it.
Delete what ever you've got in Et main that says it's etkey and generate a new key at etkey.org paste the key into ET main.
Start ET click play online, right hand under the servers it should say enable pb or disable pb make sure it's enabled, join server and alll should be fine.