Halo 2 For Vista - Not able to get an Achievement

I have just finished the First Level of Halo 2 "Cairo Station" Now there was an Achievement for it. finish this level on Normal-Legendary which I Did.

It Requires me to go online but I am unable to go online cause of the Proxy Server blocking The Live.

If i connect this computer to another internet connection and I am able to get online will I get these Achievements That the Level is already Finished?

I Already Have A GFW- Live account as AsphaltGT4 why can't I get Achievements for This Game?

TOSHIBA Satellite L500D
BIOS Version:V.1.20
OS:Windows 7 Home Edition (32-bit
CPU:AMD Turion II Ultrax2 2.50Ghz (64-bit)
GPU:1GB ATI Mobility Radeon 4650 HD Series
RAM:4GB DDR2 x86 (3.12GHz Usable)
SC:ATI HDMI Audio
MB:Realtek HDA-00
JP:Microsoft XBOX 360 Controller for Windows

Does it really matter? It's just an achievement..

Thats true, But i Like a challange

Im already signed in, on Offline Mode

Ok here is the thing, My account is registered, I played Quantam of Solace and Got the ThunderBall Achievement, it worked, another one to my list. I Have been getting Achievements for this game all week and got at least 25 of them. Why doesn't it work with Halo 2?

It Doesn't really bother me, I am just wondering why I didn't get the Achievement, unlike in QOS or Fuel when I do get achievements