Exhibit 5.1

November 4, 2010

Hewlett-Packard Company

3000 Hanover Street

Palo Alto, California 94304

Re: An aggregate of 3,642,066 Shares of Common Stock of Hewlett-Packard Company offered pursuant to the ArcSight, Inc. 2007 Equity Incentive Plan and the Amended and Restated ArcSight, Inc. 2002 Stock Plan

Dear Sir or Madam:

I have examined the proceedings taken and the instruments executed in connection with the reservation for issuance and authorization of the sale and issuance from time to time of not in excess of an aggregate of 3,642,066 shares (the “Shares”) of the Common Stock of Hewlett-Packard Company pursuant to the terms of the ArcSight, Inc. 2007 Equity Incentive Plan and the Amended and Restated ArcSight, Inc. 2002 Stock Plan (the “Plans”). The Shares are the subject of a Registration Statement on Form S-8 (the “Registration Statement”) under the Securities Act of 1933, as amended (the “Act”), which is being filed with the Securities and Exchange Commission and to which this opinion is to be attached as an exhibit.

Upon the basis of such examination, I am of the opinion that the Shares, when issued and sold pursuant to the terms and conditions set forth in the Plans and against payment therefor, and when the Registration Statement has become effective under the Act, will be validly issued, fully paid and non-assessable.

You are further advised that I consent to the filing of this opinion as an exhibit to the Registration Statement.

Very truly yours,

  /s/ PAUL T. PORRINI

Paul T. Porrini

Vice President, Deputy General Counsel

and Assistant Secretary