Several of us at my office had issues installing SQL 2008 RC0 - the installer kept failing the "Restart computer" check.
So I started digging through the registry to find the culprit flag... when this forum post turned up and pointed me to exactly the right place.
Turns out our virus scanning software always posts a pending file rename, which SQL setup interprets as the computer needing a reboot. I cleared these values and the installer ran fine.
UPDATE: Had the same problem with SQL 2008 RTM. Fix works for that too, and probably for many other installers that refuse to start due to a pending file operation.