every time i go to sign in on multi it says there is an error, and i have checked my connection and its fine and i registered my system so what do i do

sell it.

That seems to be the only way to go, sorry man. The PC version of Double Agent has many, many bugs. You could try trading in for another copy (luck of the draw and all that), but even that might not work.
I hate to sound like a hardcore SC kinda guy, but if you want online play, you might like Chaos Theory more anyways. As far as I'm concerned, and from what I've seen on Youtube, it stresses more teamwork than Double Agent and had more of what seemed appropriate for the SvM gameplay. It may be more satisfying to you in the end.