Achievements are good for what? Self-esteam?

You only need to be singed in for xbox live if you want the gamer points that are worth nothing at all and to unlock achievements and get stuff for unlocking those achievements like profile pics from the game your playing and also get goodies for your aviator from the game your playing. Otherwise you dont need to be logged in to save. But if your using the cloud to save your game then yes you have to be logged in every time. You also have to be logged in to save stuff in multiplayer modes but you will be logged in anyways to play the multiplayer mode so no worries there. Hope this helps :)

Your score is probably on Treyarch's servers and won't be reset, but maybe deleting your game save will reset it. WARNING: You will loose any progress you have made in the campaign, but you'll still keep your achievements.
To delete the save, go to the SYSTEM tab all the way right on the dashboard, Choose memory, then Games, then pick Call of Duty WaW, then pick delete. Look at any of the other folders and delete anything related to CoD World at War. Hopefully this will reset your score online.

To recover your Gamertag from Xbox LIVE:

1. Sign out from all of your profiles. The fastest way to do this is to press the Xbox Guide button, press X, and select Yes, sign out.
2. Select Recover Gamertag.
3. Follow the directions on the screen. NoteYou can enter information using a USB keyboard plugged into a controller port, the Xbox 360 Chatpad (sold as part of the Xbox 360 Messenger Kit), or your Xbox 360 controller and the on-screen virtual keyboard.
   
4. When you're finished, your profile will be downloaded from Xbox LIVE to your console and you'll see your avatar in the Sign In area. You're good to go! NoteIf you're a minor, you'll be asked to supply Windows Live ID credentials for your parent, who will need to be present to complete the procedure. Your parent will need his or her own Windows Live ID and a valid e-mail address.
   

if this happening to you
you need to recover the gamertag

1.make sure you are connected to the internet
2.logout/sign out other profiles in the console
3.press the guide button or the silver button in the middle of the controller
4.chose recover gamertag
5.enter the gamertag
6.enter the email and password ( email or windows live ID this is the email you use for the sign up of the gamertag)

um dont have the modded xbox hooked to the internet online bro, they see what that one tag does when you go online to gain ur achevements. so my point is to have ur Acheicement points show up on ur gamertag or your profile for instance, it has to be legit, that other xbox must have some file thats not allowing to use it cause that other xbox still has its internet hooked up
i think

ive had this if you are trying to do achivments always be linked to xbox live cause if your not it can muck up somtimes.

sorry, dont know about halo 3 but some achievements can only be gotten on live. and if i were you i would get a new xbox that doesnt sound good

Both consoles have advantages.
The Xbox 360 has a vast array of titles and has the biggest fan base of the two. If you connect the console to the internet, you can download games and movies through Xbox Live. You can also connect your PC to your Xbox 360 and use the console as a multimedia center using Windows Media Center.

The PS3 has a blu ray drive as standard, so you can watch blu ray movies through your PS3. DVD's are also upscaled up to 720p to improve the look of your movies. The PS3's online service is free, and allows you to download game demo's and downloadable content for games.
The Xbox 360's disadvantages are the fact you need to pay for access to Xbox Live, roughly £40 per year, but it does guarentee a reliable online service. Also earlier and cheaper models of the Xbox do not support full HD via HDMI.
The PS3 is very expensive and lacks quality titles. The console is also on a 10 year plan, so even though the console will outlast its rivals, it is likely to become outdated.
If you want to watch blu ray films and play games occasionally, go for the PS3. If you want a console with a lot of titles and a reliable online service,  get the 360. Of the two I'd choose the 360, but in 2 years or so I might need to reconsider.  

if you modified your system then it might of been banned from the live service

To recover your gamertag from Xbox LIVE:

1. Sign out from all of your profiles. The fastest way to do this is to press the Xbox Guide button, press X, and select Yes, sign out.
   
2. Select Recover Gamertag.
   
3. Follow the directions on the screen. Note
   
   You can enter information using a USB keyboard plugged into a controller port, the Xbox 360 Chatpad (sold as part of the Xbox 360 Messenger Kit), or your Xbox 360 controller and the on-screen virtual keyboard.
   
   
   
4. When you're finished, your profile will be downloaded from Xbox LIVE to your console and you'll see your avatar in the Sign In area. You're good to go!
   Note
   
   If you're a minor, you'll be asked to supply Windows Live ID credentials for your parent, who will need to be present to complete the procedure. Your parent will need his or her own Windows Live ID and a valid e-mail address.
   
   If you've forgotten your password or need help determining which e-mail address you associated with your Windows Live ID, see Xbox 360: Invalid E-mail Address or Password.
   
5. If you're recovering a Zune Tag as your gamertag, check your billing information after recovery is complete. You may need to update your billing info before you can set up an Xbox LIVE Gold membership or download content from Xbox LIVE Marketplace.