I was trying to downgrade from Diablo, (since 95% of the softwares were incompatible) so I went to the Maemo site. I clicked on "Downloads", then the "N800" under "OS & SDK". I was excited about downgrading, but I got an error. I tried this on 6 different computers and all of them failed. I also tried clearing the Cookies, History, and even disabled caching & the firewall. Please help me to get the earlier version of OS2008.

It's a problem on Nokia's end. Please see:

this, this and this (and the related bug in bugzilla: #3500)