This question comes up so often that I have an answer prepared.
An 800 MB (90 minute) disc is a nonstandard length. Any disc over 700 MB (80 minutes) must use Nero overburn settings to allow it to be burned. Use the following settings:
1. Don't use the Nero Wizard. It will not allow overburning as it defaults to Track-At-Once. If it is active, close it.
2. Multisession | No Multisession must be checked
3. ISO | Check Mode 1 and Joliet
4. Burn | Check Write and Finalize CD (No further writing possible). Write method must
5. In the main menu, File | Preferences | Expert Settings check "Enable overburn disk-at-once burning". If you are using a 800 MB data disc, you can set the Maximum CD length to 90 minutes 0 seconds 0 frm. Hit Apply, then OK. If you are using a 650 or 700 MB data disc, use the overburn capacity test in Nero CD Speed to estimate the maximum overburn capacity.
6. The Red and Yellow marker settings in File | Preferences | General Settings have no effect on burning or overburning. They are there just as a visual aid.
These settings should allow you to attempt an overburn if your recorder and the media support it.