Not at all. SQL's quite capable of handling index creation while the DB's in use.
Since you're on SQL 2005, you can create the index either online or offline. Offline is the behaviour from 2000, the table is readonly while the index is been created.
Online, the table can be read and written while the index is been created and after creation, SQL merges in any changes that happened during the build.
Either way, you don't have to force locks or transactions or anything like that. The engine handles all that internally
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