Morgan Stanley, the biggest financial brokerage, will spend $500 million to improve computer systems after flaws in a previous upgrade frustrated the staff.

Investments in technology will be made over the next 18 months, according to a memo from Greg Fleming, 49, head of its brokerage unit, that was dated yesterday and obtained by Bloomberg News. Brokers complained about glitches and more cumbersome processes last year after New York-based Morgan Stanley switched to a single system for its Smith Barney joint venture with Citigroup Inc.

“For us to be the best wealth-management firm for our advisers and clients, we must have the best platform,” Fleming wrote. “We will not stop until we have achieved this.”

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