QuickLinks -- Click here to rapidly navigate through this document

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

---

Schedule TO

Tender Offer Statement under Section 14(d)(1) or 13(e)(1)
of the Securities Exchange Act of 1934

(Amendment No. 1)

---

ARCSIGHT, INC.
(Name of Subject Company (Issuer))

Priam Acquisition Corporation
and
Hewlett-Packard Company
(Names of Filing Persons (Offerors))

---

Common Stock, par value $0.00001 per share
(Title of Class of Securities)

---

039666102
(CUSIP Number of Class of Securities)

---

Paul T. Porrini
Vice President, Deputy General Counsel & Assistant Secretary
Hewlett-Packard Company
3000 Hanover Street
Palo Alto, California 94304
(650) 857-1501
(Name, Address and Telephone Numbers of Person Authorized
to Receive Notices and Communications on Behalf of Filing Persons)

CALCULATION OF FILING FEE

*

Calculated solely for purposes of determining the filing fee. The calculation assumes (i) the purchase of 34,764,555 outstanding shares of ArcSight common stock at $43.50 per share, which is the offer price, plus (ii) $137,140,264, which is the intrinsic value of the outstanding options (i.e., the excess of $43.50 over the per share exercise price).

**

Calculated in accordance with Rule 0-11 of the Securities Exchange Act of 1934, and Fee Rate Advisory #4 for fiscal year 2010, issued December 17, 2009, by multiplying the transaction value by .00007130.

ý

Check the box if any part of the fee is offset as provided by Rule 0-11(a)(2) and identify the filing with which the offsetting fee was previously paid. Identify the previous filing by registration statement number, or the Form or Schedule and the date of its filing.

o

Check the box if the filing relates solely to preliminary communications made before the commencement of a tender offer.

Check the appropriate boxes below to designate any transactions to which the statement relates:

ý

third-party tender offer subject to Rule 14d-1.

o

issuer tender offer subject to Rule 13e-4.

o

going-private transaction subject to Rule 13e-3.

o

amendment to Schedule 13D under Rule 13d-2.

Check the following box if the filing is a final amendment reporting the results of the tender offer: o

If applicable, check the appropriate box(es) below to designate the appropriate rule provision(s) relied upon:

o

Rule 13e-4(i) (Cross-Border Issuer Tender Offer)



o

Rule 14d-1(d) (Cross-Border Third Party Tender Offer)

---

        This Amendment No.1 (this "Amendment") amends and supplements the Tender Offer Statement on Schedule TO (as amended or supplemented, the "Schedule TO") that was originally filed with the Securities and Exchange Commission on September 22, 2010 by Priam Acquisition Corporation, a Delaware corporation ("Purchaser") and a wholly-owned, direct or indirect, subsidiary of Hewlett-Packard Company, a Delaware corporation ("HP"), and HP. The Schedule TO relates to the offer by Purchaser to purchase all outstanding shares of common stock, par value $0.00001 per share (the "Shares"), of ArcSight, Inc., a Delaware corporation ("ArcSight"), at a price of $43.50 per Share, net to the seller in cash without interest thereon and less any required withholding taxes, upon terms and subject to the conditions set forth in the Offer to Purchase dated September 22, 2010 (the "Offer to Purchase") and in the related Letter of Transmittal (which together with any amendments or supplements thereto, collectively constitute the "Offer"), copies of which were filed with the Schedule TO as Exhibits (a)(1)(A) and (a)(1)(B), respectively. Capitalized terms used and not defined in this Amendment have the meanings assigned to such terms in the Offer to Purchase.

        Except as otherwise set forth below, the information set forth in the Schedule TO remains unchanged and is incorporated by reference into this Amendment.

Item 11.    Additional Information.

        (a)(3) On October 1, 2010, we received early termination of the mandatory waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (the "HSR Act"). Accordingly, the portion of the Antitrust Condition relating to the HSR Act has been satisfied. The Offer continues to be subject to the other conditions set forth in Section 14—"Conditions to the Offer" of the Offer to Purchase, including, among other things, the satisfaction of the other elements of the Antitrust Condition relating to compliance with antitrust laws in Austria and Germany and the satisfaction of the Minimum Condition.

Item 12.    Exhibits.

        Exhibit (a)(1)(B) is hereby amended by:

        On page 12 of the Form of Letter of Transmittal, deleting the last sentence in its entirety.

2

---

SIGNATURE

        After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: October 5, 2010

3

---

EXHIBIT INDEX

---

---

QuickLinks

Item 11. Additional Information.
Item 12. Exhibits.

SIGNATURE
EXHIBIT INDEX