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Usually when you install another OS on a OEM computer the recovery partition can no longer be accessed by the F-key, f11 in your case.
The Vista installation files are still there, some people have found ways to recover them, not sure about HP.
Best way is to call HP and see if they can send you the recovery DVD. Depending on their policy they should send one for free or for shipping fees.
Since OEM have the OS key stored on the computer you can find a HP recovery DVD of the same version, say Home Premium, you can use that to restore Vista to your computer.
If you cease to use the Windows 7 upgrade on a computer/OS you can legally use the previous OS again.
The OP is using a trial version of Windows 7, so the original OS license is still valid.