I think what's hapening is that as you said it should just run "Runonce" once (funny enough) but because it is coming up with a script error it never thinks it's ran and so loads everytime.
Hmmm well, if you're feeling lucky ... from the start menu, go to the 'run' command enter 'regedit' then navigate down the tree on the left to ... hkey_current_user - software - microsoft - internet explorer - main you should see quite a lot of stuff on the right, including RunonceComplete RunOnceHasShown both of these have values of 1 when the 'runonce' has finished
I've been running ie7 on 4 pcs for about 8-10 weeks and the only problems I've had are with norton 2007 crashing it on one machine until norton released a fix early last week. We have about 20 engineers here, and the majority stick with ie as it does the job and i.t. support it About half have upgraded to ie7 so far.
btw if you add the runonce values - make sure they are DWORD !