Don't send your form back to the BSA. The BSA does not have a legal right to request this information from you. Therefore, I would not return the form voluntarily. you might want to talk to a lawyer about it but we're trashing every BSA letter reminding us to submit our current list of software to them.
Alternatively, Microsoft has a tool on their website which scans a NT domain for Microsoft software and returns a list of all software used, it's called "Microsoft Software Inventory Analyzer".