Error installing SQL Server 2008 - all componets installed successfully except database engine
Make sure the account doing the installation is a member of the following groups in local security policy:


Adjust Memory quotas for a process
Bypass Traverse checking
Log on as a batch job
Log on as a service
Replace a process level token
Backup files and directories
Debug programs
Manage auditing and security log


Additional Items to check:
Make sure the account doing the install is a local admin on the server, make sure you are not installing to a compressed drive or folder, make sure you are installing the right version, IE 32 or 64 bit based on your OS