How SharePoint 2007 forms the Mysite URL (i.e /personal/firstname_lastname or /personal/domainName_firstname) ? and what is the use of the registry key "PersonalSiteURL" ?
This is configured in the Shared Service Provider (via Central Admin) in MOSS. Go to User Profiles and My Sites > My Site settings and then check out the Site Naming Format section.
You should not make any changes to the registry!