Capcom has issued a statement confirming there's a problem with the PC version of MotoGP 08.
"This issue only happens when installing the game to a custom location on a hard drive," the statement reads.
"This issue does not affect the default installation, as it only occurs when the destination location has been changed."
So, it's easy to avoid the problem. When the InstallShield Wizard asks you to "Choose Destination Location", select "Next" to accept the default directory. If you select "Change", you'll be in all sorts of trouble. Though Capcom isn't saying what kind exactly.
If that's no good to you, why not wait for the patch that will fix the problem? It's out in "the next few days", according to the Capcom statement, which concludes, "We regret any inconvenience this may have caused our MotoGP community."